Redox constituents in milk fat globule membranes and rough endoplasmic reticulum from lactating mammary gland

Comparative proteomic analysis of human milk fat globules and paired membranes and mouse milk fat globules identifies core cellular systems contributing to mammary lipid trafficking and secretion

Introduction: Human milk delivers critical nutritional and immunological support to human infants. Milk fat globules (MFGs) and their associated membranes (MFGMs) contain the majority of milk lipids and many bioactive components that contribute to neonatal development and health, yet their compositions have not been fully defined, and the mechanisms responsible for formation of these structures remain incompletely understood. Methods: In this study, we used untargeted mass spectrometry to quantitatively profile the protein compositions of freshly obtained MFGs and their paired, physically separated MFGM fractions from 13 human milk samples. We also quantitatively profiled the MFG protein compositions of 9 pooled milk samples from 18 lactating mouse dams. Results: We identified 2,453 proteins and 2,795 proteins in the majority of human MFG and MFGM samples, respectively, and 1,577 proteins in mouse MFGs. Using paired analyses of protein abundance in MFGMs compared to MFGs (MFGM-MFG; 1% FDR), we identified 699 proteins that were more highly abundant in MFGMs (MFGM-enriched), and 201 proteins that were less abundant in MFGMs (cytoplasmic). MFGM-enriched proteins comprised membrane systems (apical plasma membrane and multiple vesicular membranes) hypothesized to be responsible for lipid and protein secretion and components of membrane transport and signaling systems. Cytoplasmic proteins included ribosomal and proteasomal systems. Comparing abundance between human and mouse MFGs, we found a positive correlation (R 2 = 0.44, p < 0.0001) in the relative abundances of 1,279 proteins that were found in common across species. Discussion: Comparative pathway enrichment analyses between human and mouse samples reveal similarities in membrane trafficking and signaling pathways involved in milk fat secretion and identify potentially novel immunological components of MFGs. Our results advance knowledge of the composition and relative quantities of proteins in human and mouse MFGs in greater detail, provide a quantitative profile of specifically enriched human MFGM proteins, and identify core cellular systems involved in milk lipid secretion.

Organellar Contacts of Milk Lipid Droplets

Milk-secreting epithelial cells of the mammary gland are functionally specialized for the synthesis and secretion of large quantities of neutral lipids, a major macronutrient in milk from most mammals. Milk lipid synthesis and secretion are hormonally regulated and secretion occurs by a unique apocrine mechanism. Neutral lipids are synthesized and packaged into perilipin-2 (PLIN2) coated cytoplasmic lipid droplets within specialized cisternal domains of rough endoplasmic reticulum (ER). Continued lipid synthesis by ER membrane enzymes and lipid droplet fusion contribute to the large size of these cytoplasmic lipid droplets (5–15 μm in diameter). Lipid droplets are directionally trafficked within the epithelial cell to the apical plasma membrane. Upon contact, a molecular docking complex assembles to tether the droplet to the plasma membrane and facilitate its membrane envelopment. This docking complex consists of the transmembrane protein, butyrophilin, the cytoplasmic housekeeping protein, xanthine dehydrogenase/oxidoreductase, the lipid droplet coat proteins, PLIN2, and cell death-inducing DFFA-like effector A. Interactions of mitochondria, Golgi, and secretory vesicles with docked lipid droplets have also been reported and may supply membrane phospholipids, energy, or scaffold cytoskeleton for apocrine secretion of the lipid droplet. Final secretion of lipid droplets into the milk occurs in response to oxytocin-stimulated contraction of myoepithelial cells that surround milk-secreting epithelial cells. The mechanistic details of lipid droplet release are unknown at this time. The final secreted milk fat globule consists of a triglyceride core coated with a phospholipid monolayer and various coat proteins, fully encased in a membrane bilayer.

Milk lipid secretion: recent biomolecular aspects

Neonates of most species depend on milk lipids for calories, fat-soluble vitamins, and bioactive lipid components for growth and development during the postnatal period. To meet neonatal nutrition and development needs, the mammary gland has evolved efficient mechanisms for synthesizing and secreting large quantities of lipid during lactation. Although the biochemical steps involved in milk lipid synthesis are understood, the identities of the genes mediating these steps and the molecular physiology of milk lipid production and secretion have only recently begun to be understood in detail through advances in mouse genetics, gene expression analysis, protein structural properties, and the cell biology of lipid metabolism. This review discusses emerging data about the molecular, cellular, and structural determinants of milk lipid synthesis and secretion within the context of physiological functions.

A new approach for measuring the redox state and redox capacity in milk

Milk is one of the most fundamental ingredients in our diet. It is a complex biological fluid, which contains numerous substances, ranging from metal ions to enzymes. There is a constant search for an improved way of monitoring its quality and freshness. These are highly affected by the redox state of milk, which is governed by different species. In this study, we investigated the redox state and capacity of milk. Specifically, milk was potentiometrically titrated using different redox mediators, which enabled facilitation of electron transfer between different oxidizable species and the electrode. We found that the iodine/iodide redox couple was superior for measuring the redox capacity of milk. These measurements revealed that milk is not a well-poised system due to the presence of at least two different oxidizable species, one of which is hydrophobic while the other is hydrophilic and therefore could be separated by phase separation of milk.

Molecular determinants of milk lipid secretion

Mammary epithelial cells secrete lipids by an envelopment process that produces lipid droplets coated by membranes derived from the plasma membrane and possibly secretory vesicles. This secretion process, which resembles viral budding, is hypothesized to be mediated by specific interactions between molecules on the surface of intracellular lipids and membrane elements of the cell. Multiple lines of evidence indicate that milk lipid secretion occurs through a tripartite complex between the integral transmembrane protein, butyrophilin (BTN); the soluble metabolic enzyme, xanthine oxidoreductase (XOR); and the lipid droplet surface protein, adipophilin (ADPH). However, topological evidence from freeze-fracture replica immunolabelling (FRIL) challenge this model and suggests that milk lipid secretion is mediated by butyrophilin alone. Advances in our understanding of the molecular, structural, and functional properties of these proteins now make it possible to understand the physiological functions of each of these molecules in detail and to identify the specific molecular determinants that mediate milk lipid secretion.

The housekeeping gene xanthine oxidoreductase is necessary for milk fat droplet enveloping and secretion: gene sharing in the lactating mammary gland

Xanthine oxidoreductase (XOR) is the rate-limiting enzyme in purine catabolism occurring in most cell types. However, this housekeeping gene is expressed at very high levels in a number of mammalian tissues including the lactating mammary epithelium, suggesting additional roles for XOR in these tissues. Mice with targeted disruption of XOR were generated to assess these potential additional roles. XOR-/- mice are runted and do not live beyond 6 wk of age. Strikingly, however, XOR+/- females, although of healthy appearance and normal fertility, are unable to maintain lactation and their pups die of starvation 2 wk postpartum. Histological and whole-mount analyses showed that in XOR+/- females the mammary epithelium collapses, resulting in premature involution of the mammary gland. Electron microscopy showed that XOR is specifically required for enveloping milk fat droplets with the apical plasma membrane prior to secretion from the lactating mammary gland. We present evidence that XOR may have primarily a structural role, as a membrane-associated protein, in milk fat droplet secretion and thus XOR provides another example of "gene sharing". About 5% of women experience primary lactation insufficiency. The above observations suggest that human females suffering from xanthinuria, a deficiency in XOR, are potential candidates for lactation problems.

Nuclear coactivator protein p100 is present in endoplasmic reticulum and lipid droplets of milk secreting cells

We have identified the p100 protein, previously known as a novel cellular coactivator, as a constituent of endoplasmic reticulum and cytosolic lipid droplets from milk secreting cells. Cytosolic lipid droplets of terminally differentiated mammary epithelial cells are secreted as milk lipid globules. However, milk lipid globules did not have detectable amounts of p100 protein. The p100 protein was found also in cytosol from lactating mammary gland, in storage lipid droplets from mouse adipocytes, and in endoplasmic reticulum from liver. Immunofluorescence microscopy of mammary epithelial cells confirmed the presence of p100 in non-nuclear regions of these cells. Partial sequence analysis of tryptic peptides from p100 from cow mammary gland showed extensive homology with the reported sequence of p100 determined from a human cDNA. Antibodies against a peptide synthesized to duplicate a sequence in human p100 recognized a protein of the size of p100 in cow, mouse and rat cell fractions.

Definition of MHC-restricted CTL epitopes from non-variable number of tandem repeat sequence of MUC1

Mucin1 (MUC1) is expressed ubiquitously on breast cancer cells and is a potential target for the generation of cytotoxic T cells for vaccination against breast cancer. Thus far studies of the immunogenicity of MUC1 have used peptides from the variable number of tandem repeat (VNTR); mice so immunised can generate strong cellular and antibody responses to the VNTR of human MUC1. We now demonstrate that significant CTL and CTLp can be induced to other regions of MUC1. Using the whole native MUC1 molecule, the human milk fat globule membrane antigen (HMFG) linked to mannan, cytotoxic T cell precursors (CTLp) can be generated in BALB/c, C57BL/6, transgenic HLA-A*0201/K(b) and double transgenic HLA-A*0201/K(b)xhuman MUC1 (A2 K(b)MUC1) mice. By immunising with HMFG and testing selectively on (a) extracellular (non-VNTR); (b) VNTR and (c) intracellular peptides, it was shown that all three regions generated effective CTL. Further, the CTL responses to non-VNTR peptides were as strong as those generated to the VNTR. Epitope prediction algorithms were not particularly helpful to describe CTL epitopes: overlapping peptides had to be synthesised and tested to find the epitopes. Thus, for CTL generation, the whole HMFG molecule is a powerful immunogen when linked to mannan, especially as multiple peptide epitopes for presentation by many Class I molecules are contained within the one molecule. Furthermore, Class I restricted MUC1 CTL were generated in double transgenic A2 K(b)MUC1 mice by immunising with mannan-native mucin (HMFG), suggesting that tolerance to MUC1 can be overcome with mannan-HMFG.

A review and proposed nomenclature for major proteins of the milk-fat globule membrane

The characteristics and possible functions of the most abundant proteins associated with the bovine milk-fat globule membrane are reviewed. Under the auspices of the Milk Protein Nomenclature Committee of the ADSA, a revised nomenclature for the major membrane proteins is proposed and discussed in relation to earlier schemes. We recommend that proteins be assigned specific names as they are identified by molecular cloning and sequencing techniques. The practice of identifying proteins according to their Mr, electrophoretic mobility, or staining characteristics should be discontinued, except for uncharacterized proteins. The properties and amino acid sequences of the following proteins are discussed in detail: MUC1, xanthine dehydrogenase/oxidase, CD36, butyrophilin, adipophilin, periodic acid Schiff 6/7 (PAS 6/7), and fatty acid binding protein. In addition, a compilation of less abundant proteins associated with the bovine milk-fat globule membrane is presented.

Mouse mucin 1 (MUC1) defined by monoclonal antibodies

Mucins are highly expressed in many different human cancers and numerous murine monoclonal antibodies (MAbs) to human mucins, particularly Mucin 1 (MUC1), have been produced. However, no such antibodies to murine mucin 1 (muc1) have been described and we now describe 6 different antibodies produced to murine muc1 and to human MUC1 cytoplasmic tail, either by immunising rats, or muc1 o/o mice with synthetic peptides or a fusion protein composed of glutathione-s-transferase (GST) linked to the tandem repeat region of muc1. The antibodies to both the extracellular tandem repeat region and to the cytoplasmic tail were found to react with mucin-containing murine tissues such as breast, stomach, colon, ovary, kidney and pancreas, and the staining patterns were similar to those found in humans. The reagents reacted specifically with muc1 peptides and tissues; however, some cross reactivity with other mucin-derived peptides was noted, particularly those containing the amino acid sequence TSS. Three different epitopes (TSS, TAVLSGTS and LSGTSSP) of the M30, M70 and MFP25 MAbs were detected. Of interest was the finding that some of the antibodies reacted with murine lymphocytes; it was not clear whether these reactions were due to mucin 1 on mouse lymphocytes (MUC1 was considered to be absent from human lymphocyte), or due to cross reaction with a sialic adhesion molecule on lymphocytes. The antibodies should prove valuable reagents when studying differentiation and expression in murine glandular tissues and the ontogeny of mucin-secreting tumours.

Lipid synthesis and secretion by primary cultures of rat mammary epithelial cells

Lipid synthesis and secretion was measured in primary rat mammary epithelial cells cultured on basement matrix in medium supplemented with lactogenic hormones. The cells grew and differentiated to form alveolar-like structures reminiscent of lactating mammary gland. They synthesized abundant triacylglycerol, containing fatty acids characteristic of rat milk (C10:0-C14:0), using 14C-glucose, 14C-oleic acid or 14C-glycerol as precursors. Basal levels of triacylglycerol secretion were measured using 14C-oleic acid labeling; 1.3 +/- 0.3% of the labeled cellular triacylglycerol was secreted into the medium in 24 hours. Secreted lipid droplets were surrounded by a bilayer membrane with an electron-dense inner coat characteristic of fat globules secreted by the mammary gland. The rate of triglycerol secretion was increased to 998 +/- 98% of control (P < 0.01) by the addition of phorbol 12-myristate 13-acetate (PMA) in combination with staurosporine, a protein kinase inhibitor. Several other protein kinase inhibitors, when combined with PMA, also markedly stimulated secretion. Effective protein kinase inhibitors included sphingosine (has diverse cellular effects including the inhibition of protein kinase C; 13-fold increase in secretion), and KT5823 (a cGMP dependent protein kinase inhibitor; 5-fold increase). KT5720 (a cAMP-dependent protein kinase inhibitor) did not alter secretion. Kinase inhibitors were effective only in the presence of a phorbol ester. 4 alpha-phorbol-12,13-didecanoate, a phorbol ester which does not activate protein kinase C (PKC), could substitute for PMA. Lipid release was not mediated by disruption of cell-cell tight junctions, as EGTA did not release lipid. Based on these observations we suggest that two signals are needed to enable or stimulate lipid secretion in cultured rat mammary epithelial cells: 1) inhibition of a protein kinase and 2) a PKC-independent effect of phorbol ester. We have, for the first time, characterized a cell culture model suitable for studying lipid synthesis and secretion by mammary epithelial cells.

Effects of inositol 1,4,5-trisphosphate on calcium release from the endoplasmic reticulum and Golgi apparatus in mouse mammary epithelial cells: a comparison during pregnancy and lactation

It has been established that inositol 1,4,5-trisphosphate(IP3) is responsible for the mobilization of calcium(Ca2+) from intracellular locations in a wide variety of tissues, and that this response triggers the stimulation of several hormones and neurotransmitters. However, these phenomena have yet to be examined in the mammary epithelium. Ca2+ uptake from the medium into the endoplasmic reticulum(ER) and Golgi apparatus in vitro in both pregnant and lactating mouse mammary epithelial cells was studied and a strong Ca2+ release from these organelles into the medium with the use of IP3 was shown. The Ca2+ uptake and its release due to IP3 was also usually greater during pregnancy than lactation.

Production and characterization of a new monoclonal antibody to colorectal carcinoma

This study describes a new murine monoclonal antibody (MoAb) 5C1 raised against human colorectal carcinoma, which gave a differential reaction on formalin-fixed sections of the gastrointestinal tract. The MoAb 5C1 of immunoglobulin M (IgM) isotype reacted with both the cytoplasm and membrane of all normal colonic epithelia, and with all benign colonic polyps and all premalignant colonic lesions. However, there was a decreased expression of the 5C1 antigen in most cases of colonic malignancy and it was this feature that makes MoAb 5C1 unique. The distribution of the 5C1 epitope in normal gastrointestinal tract is limited to a few epithelial cells in the mid-portion of the small intestine but this distribution increased progressively down the digestive tract until it was found on greater than 90% of normal epithelial cells (in membrane and cytoplasm) of the colon. In addition, the 5C1 epitope was present on mucin secreting cells from normal organs of the gastrointestinal, reproductive and pulmonary tract and benign and malignant tissues of the colon. On Western blots, MoAb 5C1 was found to detect a heterogeneous population of molecules with molecular weights greater than 100 kDa with the strongest staining bands found between 230 and 300 kDa. MoAb 5C1 does not detect carcino-embryonic antigens (CEA), human milk fat globules (HMFG), human lymphocyte antigens (HLA) or ABO blood group antigens. The combination of its presence in mucin secreting cells and its broad molecular weight bands suggest that the antigen detected is a mucin.

Characteristics of a breast cancer-associated antigen defined by RCC-1 antibody

This study reports the characterization of a breast cancer-associated antigen identified by murine monoclonal antibody (MoAb) RCC-1 (formerly called 24-17.1). Immunoperoxidase staining indicated that RCC-1 recognized an antigen highly expressed in malignant tumours of breast origin, and no reactivity was noted with connective tissue, muscle or lymph nodes, which is an important consideration in its successful use in immunolymphoscintigraphy. The RCC-1 was shown to consist of 94,000 dalton disulfide-bonded dimers which were shown to be different from the transferrin receptor. In addition, the antibody RCC-1 did not react with components of human milk or with an antigenic peptide derived from the core protein of a mammary mucin. Chemical treatment and enzymatic digestion suggested that the epitope recognized by antibody RCC-1 was protein as it was resistant to neuraminidase and periodate treatment but was sensitive to trypsin. The RCC-1-defined antigen detects a novel breast cancer associated antigen.

Anti-colorectal carcinoma monoclonal antibodies reactive with human milk fat globular membranes

Two monoclonal antibodies (MoAbs) were produced against human colorectal cancer and were found to react with many colorectal carcinomas (MoAb DH-1 detected 25 of 32 and MoAb SMA-1 detected 28 of 32) by the immunoperoxidase technique. These two antibodies were found to detect epitopes present on human milk fat globules (HMFG) by enzyme-linked immunosorbent assays (ELISAs). There have been few reports of anti-mucin antibodies being produced to colon cancer and so we examined the reactions of these and 13 other anti-HMFG MoAbs with colonic cancers. Thirty-two colonic tumours were examined and different staining patterns were noted. Staining was particularly marked on the cell membranes and glandular deposits of the tumours rather than in the cytoplasm of the cells. All colonic tumours tested were positive with at least one MoAb, but no single MoAb stained all tumours; this suggested that at least one epitope of HMFG could be found on all colonic tumours but no epitope detected by any one MoAb could be found on all colonic tumours. The 15 MoAbs were also tested on a panel of adult tissues, and with the exception of two, all had unique reaction patterns. Thus, at least 13 different epitopes associated with HMFG could be detected by the panel of MoAbs based on their different tissue distributions. However, no normal tissue expressed all the epitopes detected by all the MoAbs (i.e. none was positive with all the MoAbs) and no 'monomorphic' HMFG epitope could be found. The antibodies were shown to react with carbohydrate or peptide-based epitopes, but this reactivity had no relationship with the pattern of tissue reaction and the significance of the expression of different epitopes is not clear. Preliminary results of serum tests with three different anti-HMFG MoAbs showed that approximately 10% of colorectal cancer patients had elevated HMFG levels but with the cut-off level selected, 10% of normal subjects also had raised levels. From this study, antibodies to HMFG, commonly associated with breast cancer, can also react with carcinoma of the colon and can indeed be useful in detecting this disease histologically (e.g. MoAbs CC3, 4 and 5 which detect greater than 80% of colonic tumours). In addition it is clear that some of the mucins produced in carcinoma of the colon are similar to those produced by breast tumours.

Monoclonal antibodies reactive with mucin expressed in breast cancer

Three murine monoclonal antibodies (BC1, BC2 and BC3) were developed against human milk fat globule membrane (HMFGM). By immunoperoxidase staining, it was found that the antigenic determinants had a predominant distribution in breast cancer tissue. In addition, the antibodies reacted preferentially with mucin derived from human milk rather than that derived from the breast cancer cell line ZR75; they also recognized polymorphic high molecular weight components (MW greater than or equal to 230,000) in serum and in human milk fat globule membrane. Thus the antibodies appear to react with a component of the family of mucins found in breast cancer and human milk and it appears likely that at least part of each epitope is protein in nature. Antibodies BC1, BC2 and BC3 recognized related but not identical epitopes, and they appear to be co-expressed on the same molecules as 3E1.2-defined antigen (mammary serum antigen, MSA) which is also a member of the family of breast cancer-related mucin. However, the 3E1.2 epitope is distinct and non-cross-reactive with those described for BC1, BC2 and BC3. The BC2 and BC3 defined epitopes were examined for their value in serum assays. Immunoassay was developed with a combination of two antibodies, using antibody BC3 for antigen capture and antibody BC2 or 3E1.2 for antigen detection and gave reasonable sensitivity (approximately 85%) and specificity (approximately 95%) in such serum tests for breast cancer. In a limited study, these tests appeared to complement the MSA test in the detection of breast cancer.

Purification and biochemical characterisation of a novel breast carcinoma associated mucin-like glycoprotein defined by antibody 3E1.2

A member of the high molecular weight glycoproteins of human milk and breast cancer was isolated from the sera, ascites and breast carcinoma tissue of patients with breast cancer using monoclonal antibody 3E1.2. The 3E1.2 defined antigen, termed mammary serum antigen (MSA) was obtained by immunoaffinity chromatography and a solid phase immuno-precipitation technique (SPIT) from serum of patients with metastatic breast cancer. MSA was found to be a high molecular weight glycoprotein with a Mr greater than 300,000 by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and a native Mr approximately 1 x 10(6) by gel filtration chromatography; in accord with the published Mr of other high molecular weight glycoproteins obtained from human milk and breast cancer. A high degree of glycosylation of MSA molecule was shown by its poor staining with Coomassie blue but good staining in a PAS-silver stain. In addition, MSA contained N-acetyl neuraminic acid and N-acetyl glucosamine as indicated by its binding to wheat-germ agglutinin. The epitope defined by antibody 3E1.2 is sensitive to treatment by sodium periodate and neuraminidase, implying that both carbohydrate and sialic acid are required for binding of antibody 3E1.2. Sandwich immunoassays demonstrated that MSA+ molecules are likely to express repeated 3E1.2 defined epitopes. Furthermore, MSA was susceptible to degradation by pronase, subtilisin and proteinase K and gave a different peptide profile from that of the PAS-O glycoprotein of human milk. MSA+ molecules were found to carry epitopes for a number of other monoclonal antibodies which were reactive with the PAS-O glycoprotein. It is suggested that MSA has the same core protein as is recognised by antibody DF3 which has been used to clone the same cDNA as was cloned with antibodies HMFG-1, HMFG-2 and SM-3. However, the epitope detected by the 3E1.2 antibody is either absent or weakly expressed on human milk, human milk-fat globule membrane (HMFGM) or deglycosylated HMFGM--all of which react strongly with various anti-HMFG antibodies. The antibody 3E1.2 thus recognises a unique epitope of the high molecular weight glycoproteins of human milk and breast cancer, being found in cancer tissue, serum and ascitic fluid of patients with breast cancer but weakly expressed or absent in human milk.

Isolation of plasma membranes from mammary gland by two-phase polymer partitioning

An aqueous polymer two phase partition method was adapted for isolation of plasma membranes from mammary gland of lactating rats. Plasma membranes isolated by this method were enriched 23-fold in activity of the plasma membrane marker phosphodiesterase I and were depleted, relative to homogenates, in activities of enzymes which are markers for intracellular organelles and endomembranes. Yields of plasma membranes obtained by this method were 0.3 +/- 0.1 mg protein/g wet tissue weight. Plasma membranes were isolated more rapidly and yields were more consistent than those obtained with density gradient centrifugation methods which have been applied for isolation of plasma membranes from mammary gland.

Evidence for the presence of cytochrome P-450 in mastitic bovine mammary gland

The content of cytochrome P-450 in mastitic and healthy lactating bovine mammary gland was analyzed. Evidence for the existence of cytochrome P-450 in mastitic tissues was found. The amount of cytochrome P-450 formed in the assay ranged from 0.017 to 0.031 nmole/mg protein. The highest concentrations were found in severely inflamed samples, whereas and traces or no cytochrome P-450 were found in healthy tissues. The significance of cytochrome P-450 in mastitic mammary gland is discussed in relation to host defence involving oxidative killing of pathogens.

Identification and isolation of soluble histones from bovine milk and serum

An immunoassay for soluble histones as trace components of biological fluids was developed on the basis of the dual capacity of histones to bind solid-phase DNA and monoclonal anti-histone antibody. Application of this histone-capture assay to bovine milk resulted in a positive signal, and DNA-cellulose chromatography was used to isolate histone-like material in microgram quantities. Western-blot analysis using a panel of anti-histone antibodies demonstrated the presence of histones H2A, H2B and H4 in apparently intact form. DNAase digestion experiments indicated that at least a portion of milk histone was complexed to DNA. Bovine serum was analysed in the same manner on serial DNA-cellulose columns, and H4 and partially degraded H2A were detected by Western-blot analysis. The finding of soluble histones in bovine milk and serum may account for unexpected results when these biological fluids are used as blocking reagents in Western blots and other immunoassays and may have ramifications in the origin and significance of anti-histone antibodies in human disease.

Identification of the milk fat globule membrane proteins. II. Isolation of major proteins from electrophoretic gels and comparison of their amino acid compositions

The fat globule membranes of milk are derived from the apical plasma membrane of the mammary secretory cells. The nature of the membrane proteins, as isolated from cows' milk, has been studied by the use of discontinuous and continuous SDS-gel electrophoresis. Six methods of preparation of milk fat globule membrane suggested by various authors were tested; gel electrophoresis showed that five major bands were present, independent of the method of preparation. The apparent molecular masses of these proteins as determined on SDS-gels (15% T) were 167, 142, 64, 49 and 46 kDa, respectively. The 167 kDa band stained only with periodic acid-Schiff reagent, while the 142 kDa band stained only with Coomassie blue; the last three bands stained with both. Delipidated membranes were extracted stepwise with water, 0.02 M NaCl and 0.6 M NaCl. The 64 kDa band appears to be nearly insoluble, while the bands of 142, 49 and 46 kDa are fractionated by this procedure. The resolution of all of these proteins by electrophoresis was superior to that achieved by molecular sieve chromatography, and so electrophoretic extraction was used to isolate the major proteins. Dansyl chloride derived proteins were used as markers. Amino acid compositions of the recovered proteins were obtained and are compared.

Specific neutralizing antiserum against a polypeptide growth inhibitor for mammary cells purified from bovine mammary gland

A recently published method for purification of a new inhibitor of growth of mammary cells in vitro from bovine mammary gland has been modified to yield several hundred micrograms of inhibitor per kg of glandular tissue. The inhibitory effect exerted by this preparation to Ehrlich ascites mammary carcinoma cells fulfilled all biological criteria of specificity established earlier for preparations obtained by other means, the most important being that the inhibitory effect is abolished by the epidermal growth factor and insulin. The preparation is shown to consist mainly of a protein of 13 kDa which appears to be not glycosylated. An antiserum raised in mice against the inhibitor is demonstrated to be specific for the 13 kDa bovine mammary gland protein. Neutralization of the inhibitory activity by the specific antiserum strongly supports the view that the 13 kDa protein is indeed the carrier of inhibitory activity. First data on tissue distribution obtained with an enzyme-linked immunosorbent assay revealed a high concentration of the anti-inhibitor-antiserum-reactive antigen in bovine lactating but not in non-lactating mammary gland tissue and in milk fat globule membranes. Some reactivity was also found in bovine lung. These data are interpreted with respect to a possible physiological significance of the growth inhibitor.

Dictyosome-like structures from guinea-pig testes lack galactosyltransferase, a Golgi apparatus marker

More than twenty different enzyme activities of fractions containing dictyosome-like structures (DLS) as a dominant cell component were monitored. Plasma membrane vesicles were a major contaminant of the DLS fractions, which, presumably as a consequence, were enriched somewhat in plasma membrane markers. The lysosomal enzymes arylsulfatase and latent acid phosphatase were present in the DLS fractions as were the Golgi apparatus activities thiamine pyrophosphatase and nucleoside diphosphatase. The presence of the latter two enzymes in DLS, plus NADH-ferricyanide reductase, has been verified from cytochemistry. On the other hand, the Golgi apparatus marker, galactosyltransferase, was not enriched in DLS fractions and appeared to be absent. This latter finding, verified from cytochemistry with isolated DLS fractions and, in situ, from [3H]galactose incorporation by testis tubules with analysis by autoradiography, provides the first clear biochemical characteristic that serves unequivocally to distinguish DLS from conventional Golgi apparatus.

High concentrations of antibodies to xanthine oxidase in human and animal sera. Molecular characterization

The widespread occurrence of antibodies (IgG) specific to xanthine oxidase in both normal (nonimmune) human and animal sera, and in antisera raised against a diversity of unrelated antigens is described. A study of sera from 81 humans revealed that xanthine oxidase-specific IgG represents a high proportion (1-8%) of total IgG. No obvious correlation to pathological events or symptoms of disease could be found. These xanthine oxidase-specific antibodies could be isolated by immunoaffinity chromatography on purified human or bovine xanthine oxidase and showed specific binding to the enzyme polypeptide of Mr 155,000 in immunoblotting experiments. By immunofluorescence microscopy they displayed the same cell type-specific reaction as experimentally induced antibodies, i.e., the staining of lactating mammary gland epithelium and capillary endothelium. The naturally occurring xanthine oxidase-specific antibodies consisted of polyclonal IgG of various subclasses. F(ab')2 preparations gave immune-reactions identical to those of IgG. The human xanthine oxidase-specific IgG cross-reacted with the bovine enzyme and both human and animal antibodies partially inhibited its activity. The xanthine oxidase activity of human milk lipid globules and supernatant fractions from various human tissues was extremely low when compared with that of the bovine antigen. The enzyme protein, however, was effectively precipitated from these sources by both the human and bovine antibodies. We suggest that the exceptionally high concentrations of antibodies against one protein, xanthine oxidase, are due to self-immunization to the xanthine oxidase antigen present in endothelial cells of capillaries. We do not exclude, however, nutritional contributions of bovine milk antigen to the appearance of xanthine oxidase antibodies in human sera. The possible biological functions of this immunological reaction are discussed.

Characterization of a monoclonal antibody to bovine xanthine oxidase

The isolation of a hybridoma cell line, C-41, secreting monoclonal antibody to bovine xanthine oxidase (EC 1.2.3.2), is described. The specificity of this antibody was determined by solid-phase immunoassay, immunoblotting procedures, affinity chromatography, immunoelectrophoresis and precipitation techniques. The results are compared with those obtained in similar specificity studies on a previously described monoclonal antibody secreted by hybridoma cell line A-94 [Mather, Nace, Johnson & Goldsby (1980) Biochem. J. 188, 925-928]. This latter antibody appears to bind to xanthine oxidase only when the enzyme is immobilized on a solid support such as a plastic plate or nitrocellulose paper. Potential problems in the determination of the specificity of monoclonal antibodies, especially towards membrane proteins of unknown biological activity, are discussed.

Protein synthesis in lactating guinea-pig mammary tissue perfused in vitro. I. Radiolabelling of membrane and secretory proteins

A method for the in vitro perfusion of isolated guinea-pig mammary tissue is described that allows the radiolabelling of secretory and membrane proteins. Glands were depleted of methionine, labelled with [35S]methionine for 5 min and perfused with medium containing an excess of unlabelled methionine for varying times. The structural integrity of the alveoli in the perfused glands appeared well maintained. Epithelial polarity was preserved and junctional complexes were evident. About 20% of the methionine provided in the medium was extracted by glands of 10 g wet weight under the labelling conditions employed. With chase periods from 15 to 40 min, 50-70% of the methionine was incorporated into trichloroacetic-acid (TCA)-precipitable material. The principal radiolabelled proteins recovered from the tissue fractions had Mrs and isoelectric points similar to the major secretory proteins (i.e. caseins and alpha-lactalbumin) of guinea-pig milk. Autoradiography of tissue sections at the resolution of the light microscope showed that secretory proteins were transported from sites of synthesis within secretory cells to the alveolar lumina after 45 min. These highly labelled secretory proteins could be almost completely removed from microsomal fractions by treatment with sodium carbonate solutions. Proteins with Mrs from 30 000 to 200 000 were detected in the washed membranes by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and fluorography. These labelled membrane-associated proteins persisted in the microsomal membrane fraction after chase periods from 7.5 to 40 min.

Interaction of antimycin with cytochrome b-561. A study in secretory granules and in plasma membrane isolated from chromaffin cells of bovine adrenal medulla

Cytochrome b-561 in chromaffin granules interacts with antimycin and its alpha-peak shifts 1 nm towards red. When chromaffin granules were treated with Triton X-100 antimycin no effect was observed. Cytochrome b-561 is located in the plasma membrane isolated from the chromaffin cells. The plasma membrane b-561 does not seem to interact with antimycin. A number of NADH or NADPH (acceptor) oxidoreductase activity has been observed in isolated plasma membrane providing clues to the origin of plasma membrane dehydrogenase. The possible role of cytochrome b-561 in secretory granules other than its accredited energy conserving electron transport property is projected.

Biochemical and immunological characterization of desmoplakins I and II, the major polypeptides of the desmosomal plaque

Epithelial cells contain complexes of cytokeratin filaments (tonofilaments) with specific domains of the plasma membrane that appear as symmetric junctions, i.e. desmosomes, or as asymmetric hemi-desmosomes. These regions of filament-membrane-attachment are characterized by 14 to 20 nm thick dense plaques (desmosomal plaque). In isolated desmosome-tonofilament complexes or other desmosomal fractions from various stratified squamous epithelia (e.g. bovine muzzle epidermis and tongue mucosa) desmosomal plaque structures are recognized and show a relatively high resistance to various extraction buffers and detergents. Such fractions enriched in desmosomal plaque material are also enriched in two prominent polypeptide bands of apparent molecular weights 250,000 (desmoplakin I) and 215,000 (desmoplakin II) which appear, on two-dimensional gel electrophoresis, as two distinct polypeptides isoelectric near neutral pH. These two polypeptides are present in almost equimolar amounts and each of them appears as a series of isoelectric variants, including some labeled by [32P]phosphate in tissue slices. The two desmoplakin polypeptides are closely related as shown by tryptic peptide map analysis and are different from keratin-like proteins and other major polypeptides of desmosome-rich fractions. Guinea pig antibodies raised against desmoplakins and specific for these proteins do not cross-react with other desmosomal antigen(s) or constituents of other types of junctions. Using desmoplakin antibodies we have identified desmoplakins as the major constituents of the desmosomal plaques present in epithelial and myocardiac cells of diverse species. The significance of this group of cell type-specific membrane-associated cytoskeletal proteins and their possible cytoskeletal functions are discussed.

Butyrophilin, an apical plasma membrane-associated glycoprotein characteristic of lactating mammary glands of diverse species

Lipid globule membranes were isolated from human and bovine milk and from the milk of sheep, goat, pig, rat and guinea pig, and their polypeptide compositions were analyzed. The major polypeptides with molecular weights similar to that of bovine butyrophilin were separated by gel electrophoresis, isolated and characterized with respect to isoelectric point, molecular weight, immunological cross-reactivity and peptide composition after proteolytic cleavage. We show that in all species examined these proteins are similar to bovine butyrophilin in (i) their relative insolubility in buffers of low and high ionic strength and in non-denaturing detergents, (ii) the occurrence of several isoelectric variants, and (iii) patterns of peptides obtained by protease digestion. It is concluded that closely related proteins are major constituents of the cytoplasmic coat structures associated with milk lipid globule membranes of many species, and we propose the name butyrophilins for this group of proteins. Bovine and human butyrophilins are glycosylated with relatively large amounts of glucosamine, mannose, glucose and galactose but little fucose, sialic acids or galactosamine. Most if not all of the sugar residues are associated with an acetone-soluble peptide fragment of Mr 12000-16000 focusing at about pH 4.0. We suggest that this fragment contains a membrane-spanning peptide sequence and is involved in the attachment of the cytoplasmic coat to the membrane of the milk lipid globule.

Differential scanning calorimetry of milk fat globule membranes

Differential scanning calorimetry was employed as an aid in examining the structure of the bovine milk fat globule membrane. At least six major endotherms are observed between 10 and 90 degree C, corresponding to order-disorder transitions of discrete structural domains of the membrane. These endothermic transitions occur at 16, 28, 43, 58, 68, and 75 degrees C. The transitions occurring between 10 and 50 degrees C were reversible, suggesting the involvement of lipid. However, the high temperature transitions were irreversible. The calorimetric C transition, center at 43 degree C, was shown to involve neutral lipid, since the endotherm was reversible, insensitive to proteolysis, and similar to the endotherm of the isolated neutral lipid fraction of the milk fat globule membrane. The glycolipid and phospholipid fractions of the milk fat globule membrane yielded endotherms outside of the temperature range of the C transition. Another endotherm, the D transition (58 degree C), was found to involve the denaturation of the major membrane coat protein, butyrophilin (band 12). Evidence for this assignment included the following observations: (i) the nearly selective proteolysis of butyrophilin resulted in the complete removal of the D transition, (ii) the butyrophilin-enriched, Triton X-100-insoluble pellet of milk fat globule membrane yielded a relatively normal D transition, and (iii) the irreversible, disulfide-stabilized aggregation of butyrophilin occurred in the membrane solely at the temperature of the D transition. Furthermore, no other prominent milk fat globule membrane polypeptide formed these non-native disulfide crossbridges during the D transition. The sources of the other major endotherms of the milk fat globule membrane have not yet been assigned.

Iron and copper in plasma membranes

Nonheme iron has been found in pig erythrocyte and mouse liver plasma membranes. The amount found, 8.2 nmol/mg protein in erythrocyte membranes and 7.4 nmol/mg protein in liver plasma membrane, is slightly lower than values reported for endoplasmic reticulum and Golgi apparatus. Less than one-third of the erythrocyte membrane iron can be released by acid treatment, which indicates that most of it is not in the typical iron-sulfur structure. Copper has been found in pig erythrocyte plasma membrane at a concentration of 0.45 nmol/mg protein. These metals may be associated with the redox enzymes of plasma membranes.

Stearoyl-coenzyme A desaturase activity in the mammary gland and liver of lactating rats

Stearoyl-CoA desaturase activity in microsomes from lactating rat mammary gland is very low (0.05-0.15 nmol/min/mg of protein) regardless of lactating time. In such microsomes, reductase activities and content of cytochrome b5 are several-fold lower than in normal rat liver microsomes. Preincubation of the mammary microsomes with purified terminal desaturase gives a 55-fold stimulation of stearoyl-CoA desaturase activity, whereas preincubation with cytochrome b5 has no effect. However, preincubation of mammary microsomes with both cytochrome b5 and terminal desaturase results in a 200-fold stimulation of overall desaturation. These observations suggest that negligible stearoyl-CoA desaturase activity in lactating rat mammary microsomes is due to a cytochrome b5 content and the absence of terminal enzyme. The hepatic stearoyl-CoA desaturase activity increases 9-fold during lactation. There is little or no change in the NADH-cytochrome c reductase activity or in the concentrations of cytochrome b5 during this period, but the activity of the terminal desaturase increases with the increase of overall desaturation. These results suggest that liver is one of the more important sources of oleic acid for milk triglycerides.

Characteristics of membrane-bound and soluble forms of xanthine oxidase from milk and endothelial cells of capillaries

Xanthine oxidase (xanthine:O2 oxidoreductase, EC 1.2.3.2) was purified from bovine milk lipid globules to electrophoretic homogeneity (Mr 155,000) and antibodies were raised against it in rabbits. By immunolocalization techniques, the xanthine oxidase antigen was detected in milk lipid globules and mammary gland epithelium, but also in capillary endothelium from various tissues, including liver, lung and intestine. These findings were paralleled by measurements of xanthine oxidase activities in the tissues, both in a membrane-associated and a soluble form. Addition of hypoxanthine to fractions containing native xanthine oxidase did not promote lipid peroxidation, in contrast to the widely used in vitro system for lipid peroxidation which involves addition of xanthine oxidase preparations. Extraction with buffers of high ionic strength and with nonionic detergents removed only part of the enzyme from the membranes. Immunoprecipitates from the soluble supernatant fractions, using anti-xanthine oxidase IgG, were enriched in the Mr 155,000 polypeptide. Patterns of proteolytic cleavage products of the xanthine oxidase monomer from capillaries and milk lipid globules were similar but not identical. Immunoprecipitates from soluble fractions of milk lipid globules and tissues were enriched in both xanthine oxidase and NADH-cytochrome c reductase activities. Electrophoretic separation of proteins from milk lipid globule membranes under non-denaturing conditions revealed a close correlation of xanthine oxidase and part of the NADH-cytochrome c reductase activity, but showed different activity profiles of NADH-ferricyanide reductase and xanthine oxidase.

Detection of xanthine oxidase and immunologically related proteins in fractions from bovine mammary tissue and milk after electrophoresis in polyacrylamide gels containing sodium dodecyl sulphate

A solid-phase immunoassay was used to detect xanthine oxidase in fractions from bovine mammary glands after electrophoresis in polyacrylamide gels containing sodium dodecyl sulphate. Under these conditions the major proportion of xanthine oxidase in either mammary tissue or mild could be recovered as a protein of mol.wt. 150 000. In mammary tissue approx. 80% of the enzyme was in a soluble form and the remainder was accounted for in either 'mitochondrial' or microsomal fractions after tissue homogenization and fractionation. Affinity chromatography of either detergent-solubilized microsomal membranes or postmicrosomal supernatants on immobilized antibody to xanthine oxidase yielded a single protein that cross-reacted with antibody to the enzyme. In milk presumptive degradation products of the enzyme were detected in minor quantities with mol.wts. of 43 000 in the whey fraction and 90 000 in fat-globule membrane. Only the undegraded enzyme was present in the skim-milk membrane fraction. Xanthine oxidase is therefore synthesized and secreted as a protein with a monomeric mol.wt. of 150 000 and is not subjected to extensive proteolytic degradation during the storage of milk in mammary alveoli. The significance of the results is discussed in relation to the overall protein composition of the membranes of milk-fat globules and skim milk.

Mixed function oxidase in the mammary gland and liver microsomes of lactating rats. Effects of 3-methylcholanthrene and beta-naphthoflavone

Mammary gland and liver microsomes of lactating rats were examined for the components of mixed function oxidase and related enzyme activities. Cytochrome b5, NADH- and NADPH- dependent cytochrome c reductase activities were 15-, 6- and 10-fold lower, respectively, in the mammary gland than in the liver microsomes. The determination of cytochrome P-450 (P-448) in the mammary gland microsomes required elimination of the spectral interferences by hemoglobin and cytochrome aa3. The presence of the latter in this fraction was also shown by cytochrome c oxidase activity. Cytochrome aa3 was reduced by anaerobic incubation of mammary gland microsomes, in the presence of antimycin A, with sodium succinate, phenazine ethosulfate, and sodium ascorbate for 30 min at room temperature. Spectral resolution of the dithionite-reduced cytochrome P-450 (P-488) carbon monoxide complex occurred 30 min after gassing. The basal level of cytochrome P-450 was about 500-fold greater in the liver than in the mammary gland microsomes. Pretreatment of lactating rats with the inducers of hepatic cytochrome P-448, 3-methylcholanthrene and beta-naphthoflavone, increased the cytochrome content 3- to 10-fold, in the mammary gland and liver microsomes, respectively. The induction of cytochrome P-448 in microsomes of both tissues was also shown by type I binding spectra obtained with N-2-fluorenylacetamide. Using hydroxylation of benzo[a]pyrene and N-2-fluorenylacetamide as a measure of mixed function oxidase activity, we found that the basal activities, which were 4- to 8-fold greater in the liver microsomes, were increased in both tissues after treatment of rats with the inducers. The induced activities were inhibited by 0.1 micrometers alpha-napthoflavone in vitro, indicating a dependence on cytochrome P-448. The data suggest that the mammary gland, an extrahepatic target for carcinogens, is capable of their metabolism.

Antibodies to high molecular weight polypeptides of desmosomes: specific localization of a class of junctional proteins in cells and tissue

Desmosome-enriched fractions were isolated from bovine muzzle epidermis either as desmosome-tonofilament complexes using a procedure involving treatment at pH9 or in the form of desmosomal residue fractions using a modification of the citric acid buffer (pH 2.3) method of Skerrow and Matoltsy [1]. Major polypeptides of high molecular weights (mol. wt.) were separated by gel electrophoresis, individual polypeptide bands were excised, and protein was eluted and used for immunization. Guinea pig antibodies raised against two prominent polypeptides of high mol. wt. (250,000 and 215,000) showed, on nitrocellulose paper blots of desmosome-tonofilament polypeptides separated by gel electrophoresis, extensive cross-reaction between a group or large polypeptides characteristic of desmosome-containing fractions, most notably polypeptides of 250 K, 215 K, 200 k, 175 K, and 164 K. These antibodies allowed, when used in immunofluorescence microscopy, the specific localization of desmosomal junctions (i) in sections through epithelia-containing tissue (e.g., epidermis, mucosae of tongue and esophagus, cornea, mammary gland, small intestine, liver, thymus, urothelium of bladder) and myocardium; (ii) on dissociated cells from these tissues; (iii) on various epithelial cells grown in culture; an (iv) in tumor-like proliferations of cultured epithelial cells injected into nude mice. Individual desmosomes could be visualized and resolved at the light microscopic level. No reaction was found in cells devoid of desmosomes and on other classes of intercellular junctions. Electron microscopic localization using immunoperoxidase techniques indicated that these proteins are located in, or close to, the desmosomal plague structure. It is proposed to use such antibodies against desmosomal proteins as markers specific to this so far only morphologically define class of junctions. Use of these markers will (i) improve identification and classification of intercellular junctions; (ii) facilitate determinations of the specific patterns of distributions of desmosomes and desmosomal protein in various cells and tissue; (iii) allow studies of formation and disintegration of desmosomes, and of the biosynthesis and possible recycling of their constituents; and (iv) provide tissue group-specific markers valuable in histology and diagnosis, especially for identification of epithelial and carcinoma cells.

Localization of xanthine oxidase in mammary-gland epithelium and capillary endothelium

Xanthine oxidase, an iron-sulfur molybdenum flavoprotein known to generate superoxide radical, was demonstrated in several bovine tissues. The enzyme (155 kd polypeptide) was purified from bovine milk lipid globules and antibodies were raised that allowed precipitation of the enzyme without inactivation of enzymatic activity. By immunolocalization techniques at light and electron microscope levels, the antigen was found in milk-secreting epithelial cells but not in epithelial cells of several other tissues. In a number of tissues, including mammary gland, liver, heart, lung and intestine, antibodies to xanthine oxidase stained only endothelial cells of capillaries, including sinusoids, but not endothelia of larger blood vessels and endocard. In both milk-secreting epithelial and capillary endothelial cells, xanthine oxidase was distributed throughout the cytoplasm. Results from biochemical and immunological studies suggest that xanthine oxidase is similar in the various tissues examined and may serve similar redox functions.

Antibodies to the major insoluble milk fat globule membrane-associated protein: specific location in apical regions of lactating epithelial cells

Milk lipid globules of various species are surrounded by a membrane structure that is separated from the triglyceride core of the globule by a densely staining fuzzy coat layer of 10- to 50-nm thickness. This internal coat structure remains attached to the membrane during isolation and extraction with low- and high-salt buffers, is insoluble in nondenaturing detergents, and is enriched in an acidic glycoprotein (butyrophilin) with an apparent Mr of 67,000. Guinea pig antibodies against this protein, which show cross-reaction with the corresponding protein in some (goat) but not other (human, rat) species, have been used for localization of butyrophilin on frozen sections of various tissues from cow by immunofluorescence and electron microscopy. Significant reaction is found only in milk-secreting epithelial cells and not in other cell types of mammary gland and various epithelial tissues. In milk-secreting cells, the staining is restricted to the apical cell surface, including budding milk lipid globules, and to the periphery of the milk lipid globules contained in the alveolar lumina. These findings indicate that butyrophilin, which is constitutively secreted by surface budding in coordination with milk lipid production, is located at the apical surface and is not detected at basolateral surfaces, in endoplasmic reticulum, and in Golgi apparatus. This protein structure represents an example of a cell type-specific cytoskeletal component in a cell apex. It is suggested that this antigen provides a specific marker for the apical surface of milk-secreting cells and that butyrophilin is involved in the vectorial discharge of milk lipid globules.

Plasma membranes from intestinal microvilli and erythrocytes contain cytochromes b5 and P-420

The presence of cytochromes b5, P-450 and P-420 and activities of NADH- and NADPH-cytochrome c redutases were determined in plasma membranes isolated from microvilli of the chick and rat intestinal epithelium and erythrocyte membranes from chick, rat and man. The results are compared with the amounts of these components found in microsomal fractions from intestinal epithelium and in nuclear membranes from chick erythrocytes. Plasma membranes from intestinal microvilli and from erythrocytes contained significant amounts of NADH-cytochrome c reductase activity and of a pigment spectrophotometrically indistinguishable from rat liver microsomal cytochrome b5. In addition, cytochrome b5 fragments were prepared from the membranes by limited trypsin digestion and consisted of two to four components with Mr values in the range 10 000-13 500. In low-temperature difference spectra, the presence of a second cytochrome was noted which was similar to cytochrome P-420. Cytochrome P-450 and NADPH-cytochrome c reductase activities were not detected in plasma membrane fractions in significant concentrations but were present in the corresponding endomembrane fractions. These findings in highly purified, well defined plasma membrane fractions, in which contamination by endomembranes is minimal, strengthen the evidence for the existence of cytochrome-containing redox systems in plasma membranes of various cells and suggest that such redox components are general components of the cell surface. Possible functions and origins of these redox components in plasma membranes are discussed.

Intermediate-sized filaments of the prekeratin type in myoepithelial cells

Myoepithelial cells from mammary glands, the modified sweat glands of bovine muzzle, and salivary glands have been studied by electron microscopy and by immunofluorescence microscopy in frozen sections in an attempt to further characterize the type of intermediate-sized filaments present in these cells. Electron microscopy has shown that all myoepithelial cells contain extensive meshworks of intermediate-sized (7--11-nm) filaments, many of which are anchored at typical desmosomes or hemidesmosomes. The intermediate-sized filaments are also intimately associated with masses of contractile elements, identified as bundles of typical 5--6-nm microfilaments and with characteristically spaced dense bodies. This organization resembles that described for various smooth muscle cells. In immunofluorescence microscopy, using antibodies specific for the various classes of intermediate-sized filaments, the myoepithelial cells are strongly decorated by antibodies to prekeratin. They are not specifically stained by antibodies to vimentin, which stain mesenchymal cells, nor by antibodies to chick gizzard desmin, which decorate fibrils in smooth muscle Z bands and intercalated disks in skeletal and cardiac muscle of mammals. Myoepithelial cells are also strongly stained by antibodies to actin. The observations show (a) that the epithelial character, as indicated by the presence of intermediate-sized filaments of the prekeratin type, is maintained in the differentiated contractile myoepithelial cell, and (b) that desmin and desmin-containing filaments are not generally associated with musclelike cell specialization for contraction but are specific to myogenic differentiation. The data also suggest that in myoepithelial cells prekeratin filaments are arranged--and might function--in a manner similar to the desmin filaments in smooth muscle cells.