Prostaglandin and thromboxane synthesis by tissue slices from human aortic aneurysms

Sliced portions of the walls of human aortic aneurysms were incubated with extracts of human plasma and serum to determine the profile of prostanoid production. 6-Oxo-prostaglandin (PG) F1 alpha, PGE2, PGF2 alpha, and thromboxane (TX) B2 were measured by gas chromatography/electron capture mass spectrometry. 6-Oxo-PGF1 alpha, the stable hydrolysis product of PGI2, was the major cyclooxygenase product but substantial amounts of TXB2 (the hydrolysis product of TXA2), with smaller amounts of PGE2 and PGF2 alpha were also synthesised. These prostanoids could contribute to the response of the vascular wall to injury, thereby influencing the disease process. Serum extracts stimulated PGI2 and TXA2 synthesis, probably as a result of their Ca2+ content.

DNA polymorphisms of the gene for apolipoprotein B in patients with peripheral arterial disease

We have determined the frequency of DNA polymorphisms of the gene for human apolipoprotein B, detected with XbaI and EcoRI, in 205 patients with documented peripheral arterial disease. Of the patients, 78 have no evidence of disease in the coronary and carotid arteries, 64 have coexisting coronary artery disease but no evidence of carotid artery disease, 26 patients have coexisting carotid artery disease but no evidence of coronary artery disease, and 37 have coexisting coronary and carotid artery disease. Levels of triglycerides, cholesterol and apolipoprotein B were measured for each patient, and RFLP frequency was determined in all the patients. Lipid, lipoprotein and apolipoprotein levels were not significantly different between the different patient groups. Compared with a sample from the clinically well London population, the frequency of the R2 allele of the polymorphism detected with EcoRI, and the frequency of the X1 allele of the XbaI polymorphism was significantly higher in the patient group. The frequency of these alleles was not significantly different in the different patient groups. In patients with only peripheral arterial disease, individuals with the XbaI genotype X1X1 have the lowest and those with the genotype X2X2 have the highest mean levels of serum cholesterol. However, in all other patient groups this trend was reversed (X1X1 highest and X2X2 lowest). Our observations suggest that variation at the apo B locus is one of the factors involved in predisposing an individual to develop arterial disease but does not determine where in the arterial system the disease develops.

Collagen in abdominal aortic aneurysm: typing, content, and degradation

The collagen content of the media of infrarenal aorta has been compared in age- and sex-matched normal aorta and dilated and nondilated atherosclerotic aorta. The proportion of collagen was increased in aneurysmal aorta from 62% to 84% and appears to be the result of preferential elastin degradation. The ratio of type I to type III collagen, estimated from sodium dodecyl sulfate-polyacrylamide gel electrophoresis of specific cyanogenbromide peptides, did not vary significantly from 2:1 in any of the three groups of aortas. There was no evidence of increased collagenase activity in unruptured aneurysmal aorta. Collagenase activity was increased in ruptured aneurysmal aorta but could only be satisfactorily measured after resolution from the endogenous tissue inhibitor of metalloproteinases. We suggest that only limited collagen turnover occurs in the media of abdominal aortic aneurysms before rupture. A subgroup of three patients with a significant family history of aneurysm had lower amounts of type III collagen in the aortic media, suggesting that abnormalities in type III collagen may be one of the genetic factors contributing to familial clustering of aneurysms.

Acute phase proteins in patients with abdominal aortic aneurysms

Both stenosis and aneurysmal dilatation are associated with atherosclerosis of the distal aorta. As part of an investigation into factors predisposing to aneurysmal dilatation we compared the levels of acute phase proteins in patients with stenosing and aneurysmal disease. Increased levels of acute phase proteins were found in patients with abdominal aortic aneurysms compared with patients with stenosing aortic disease. In 20 aneurysm patients the C-reactive protein was 56 +/- 10 mg/l, alpha 1-proteinase inhibitor 2.5 +/- 0.13 g/l and ceruloplasmin 0.41 +/- 0.01 g/l. In 20 patients with stenosing aortic disease the C-reactive protein was 28 +/- 8 mg/l, alpha 1-proteinase inhibitor 1.65 +/- 0.11 g/l and ceruloplasmin 0.35 +/- 0.03 g/l. These results argue for the participation of an inflammatory process in the aortic wall in the pathogenesis of all aneurysms.

Elastin degradation in abdominal aortic aneurysms

Histological sections through the walls of abdominal aortic aneurysms showed scarce and disrupted elastic tissue. The elastin content of the aneurysmal aortic media was only 8.1 +/- 3.2% dry defatted weight (n = 11). The elastin content of grossly normal age and anatomically matched aortic media was 35.0 +/- 3.2% dry weight (n = 4) and the elastin content of severely atherosclerotic, stenosed infrarenal aortic media was 22.0 +/- 7.2% dry weight (n = 6). There was an inverse correlation of elastin content with the elastinolytic activity of aortic media homogenates, r = -0.78. Elastase activity, measured by the hydrolysis of [3H]elastin, was highest in aneurysmal aortic homogenates, 92.1 +/- 43.7 U/mg protein (n = 18), falling to 46.9 +/- 13.3 U/mg protein (n = 13) in severely stenosed atherosclerotic aortic homogenates and 35.5 +/- 11.9 U/mg (n = 6) in grossly normal aortic homogenates. The elastinolytic activity of stenotic aorta contained leukocyte elastase as an important component. In aneurysmal homogenates leukocyte elastase was also found but the increased elastase activity resulted from a protease(s) (Mr 95,000) extracted in 2 M urea, having minimal specificity for alanyl bonds and no immunological cross-reactivity with leukocyte elastase.

Multifactorial inheritance of abdominal aortic aneurysm

The possible genetic predisposition to abdominal aortic aneurysm was investigated by recording the family history from 60 consecutive patients presenting for aneurysm repair. Twenty patients, age 63.2 +/- 9.3 years, had at least one first degree relative with abdominal aortic aneurysm. Thirty six patients, age 69.6 +/- 6.3 years, knew of no aortic aneurysm amongst first degree relatives and four patients could not give a full family history. A total of 25 among 320 first degree relatives (8%) or 14 amongst 192 siblings (7%) had an abdominal aortic aneurysm. Both the younger age of presentation (P less than 0.01) and the concentration of women amongst those with a positive family history argue for multifactorial inheritance of abdominal aortic aneurysm, with the genetic component calculated as approximately 70%. These same factors suggest that it might be worthwhile screening for aneurysmal disease the sibling smokers of female and younger male probands.

A quantitative assay for the adhesion of alveolar type II cells: application to the study of epithelial-fibroblast interactions

A quantitative assay for the adhesion of rat lung epithelial cells has been developed. Cells are adhered to 96 well tissue culture plates and quantitated by their reaction with monoclonal antibodies to cytokeratin in an enzyme linked assay. Using this technique we have shown that rat lung alveolar Type II cells preferentially adhere to a fibroblast monolayer. There was no preferential adhesion of epithelial cells to fibroblast secreted matrix or to the separate components of alveolar matrix.

Oxidation and chemical modification of lung beta-galactoside-specific lectin

Galaptins are small, soluble, lectins with a specificity for beta-galactose residues. Many galaptins are inactivated by atmospheric oxygen and are protected by disulphide-reducing reagents. We find that each subunit of rat lung galaptin contains one residue of tryptophan and six of cysteine. Oxygen inactivates rat lung galaptin by oxidation of the cysteine residues. During oxidation, the normal dimeric structure is maintained and all disulphide bonds are formed within individual subunits. Exogenous thiols protect against inactivation, but oxidized thiols accelerate inactivation. Human lung fibroblast galaptin is almost completely inactivated within 1 h in tissue culture medium at 37 degrees C. Alkylation of native rat lung galaptin with iodoacetate or ethyleneimine causes substantial loss of activity. The dimeric galaptin structure is maintained. In contrast, alkylation with iodoacetamide yields carboxamidomethyl-galaptin, which is fully active and stable to atmospheric oxygen in the absence of disulphide-reducing reagents. This derivative is very useful for studies of galaptin properties and function.

Opsonic dysfunction secondary to plasma fibronectin depletion after aortic surgery

Fibronectin is a plasma opsonin which is depleted after major injury or surgery. The effect of major aortic surgery on plasma fibronectin has been studied in patients and in an equivalent experimental model in pigs. On the day after aortic surgery in eleven patients mean fibronectin (+/- s.e.m.) had fallen from 363 +/- 11 mg/l pre-operatively to 179 +/- 19 mg/l (P less than 0.01). No such fall was observed in patients undergoing herniorrhaphy. In fourteen pigs aortic surgery produced reproducible surgical shock and a fall in plasma fibronectin from 331 +/- 10 mg/l to 43 +/- 13 mg/l after resuscitation (P less than 0.01). Whenever plasma fibronectin fell below 190 mg/l the circulating free fibronectin was consumed in complexes of 1000 kDa containing collagenous debris. More severe depletion of plasma fibronectin was related to higher concentrations of circulating nonopsonized collagenous debris and to subsequent mortality in pigs. The depletion of free fibronectin that occurs following major surgery may produce clinically important opsonic dysfunction. The clinical relevance of this fibronectin consumption may be missed if measurement is limited to circulating fibronectin levels without determining that proportion bound in complexes and no longer available as an opsonin.

Chemical modification of arginine residues of lung galaptin and fibronectin. Effects on fibroblast binding

Lung galaptin bound to lung fibroblasts with a Kd of 190 nM, and this binding could be inhibited by 20 mM-lactose. Selective modifications of the arginine residues of galaptin with cyclohexane-1,2-dione did not change its lectin activity or its binding to fibroblasts. By contrast, modification of the arginine residues of plasma fibronectin resulted in a marked diminution of protein-fibroblast binding. Selective modification of arginine residues may provide a useful probe for -Arg-Gly-Asp-Xaa cell-binding sequences of proteins.

Endogenous ligands of rat lung beta-galactoside-binding protein (galaptin) isolated by affinity chromatography on carboxyamidomethylated-galaptin-Sepharose

Rat lung beta-galactoside-binding protein (galaptin) is developmentally regulated during postnatal lung development. In common with other vertebrate galaptins, it is very labile when purified and dependent on the presence of exogenous thiol reagents. Reaction of rat lung galaptin with iodoacetamide resulted in a stable active carboxyamidomethylated galaptin that could be coupled to Sepharose. The resultant affinity matrix bound asialoglycoproteins, and these could be quantitatively eluted with disaccharide haptens. The carboxyamidomethylated-galaptin-Sepharose affinity matrix was used to search for endogenous ligands in 13-day-rat lung. Cytosolic fractions of developing rat lung contained no moieties that could be specifically eluted with disaccharide hapten. Only when membranous fractions were extracted with 1% Triton were glycoproteins solubilized that bound to the affinity matrix and could be specifically eluted with disaccharide hapten. The eluted glycoproteins were potent inhibitors of galaptin binding to asialo-orosomucoid. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis identified these glycoproteins as being of high Mr, with three components of Mr 160000-200000 and a smaller component of Mr 75000. This is the first evidence for specific membrane-associated glycoproteins being the ligands of rat lung galaptin.

The subcellular localization of the beta-galactoside-binding protein of rat lung

The subcellular localization of the beta-galactoside-binding protein, or lectin, from rat lung was investigated by the specific binding of anti-lectin immunoglobulin G to subcellular fractions. We used both adult and immature (12-day-old) rats; the immature rat lungs have an 8-10-fold greater concentration than adult rat lungs [Powell & Whitney (1980) Biochem. J. 188, 1-8]. In both groups of animals we observed greater specific binding of anti-lectin immunoglobulin G to intracellular membrane (mitochondrial and microsomal fractions) than to plasma membranes. Pre-incubation of membrane fractions with lactose resulted in a marked diminution of anti-lectin immunoglobulin G binding. In the adult rat lung most (approx. 80%) of the lectin activity was membrane-associated. In the immature rat lung only approx. 30% of the lectin activity was membrane associated and most of the beta-galactoside-binding protein appeared to be a soluble cytoplasmic component. The rat lung beta-galactoside-binding protein appeared to have a broad but predominantly intracellular location, being associated with membranes through one of its galactoside-binding sites.

Postnatal development of rat lung. Changes in lung lectin, elastin, acetylcholinesterase and other enzymes

The development of rat lung from a primitive gas-exchange organ to the mature respiratory organ is in large part a postnatal phenomenon that has been well characterized by morphological and morphometric methods. The alveolarization of the lung is achieved during the first 3 weeks of life. Cholinergic innervation of rat lung also appears postnatally. We have monitored the presence or activity of several proteins during postnatal rat lung development. Newborn-rat lung contains negligible amounts of acetylcholinesterase, but the specific activity of acetylcholinesterase reaches adult values by postnatal day 10-11. Neonatal-rat lung does not contain significant amounts of beta-galactoside-binding protein [Powell (1980) Biochem. J.187, 123-129]. The activity of this endogenous lung lectin was apparent at about day 6, was maximal between days 10 and 13 before declining 8-10-fold to reach adult values. Elastin has been implicated from morphological evidence as critical to lung restructuring. We have quantified the amount of desmosine and isodesmosine per g wet wt. of lung. The concentration of elastin, by this criterion, was low and stationary until postnatal day 7; a dramatic increase in elastin concentration occurred between days 10 and 20, when adult values were reached. The peak of lung-lectin activity was coincident with the maturation of acetylcholinesterase and the beginning of rapid elastin cross-linking. The specific activities of angiotensin-converting enzyme, carbonic anhydrase, choline kinase and glucose 6-phosphate dehydrogenase were also monitored.

Purification and properties of lung lectin. Rat lung and human lung beta-galactoside-binding proteins

Lung is one of the organs of the rat with a particular abundance of haemagglutinating activity that is inhibited by beta-galactosides. This lectin activity can be attributed to a single protein that has been purified from rat lung; a similar protein has been purified from human lung. The molecular weights and subunit structures were estimated from gel filtration and sodium dodecyl sulphate/polyacrylamide-gel electrophoresis; the human lung lectin appeared to be composed to two identical subunits, mol.wt. 14500, whereas rat lung lectin was observed as both a dimer and a tetramer of one subunit type, mol.wt. 13500. Both lectins bind to disaccharides or oligosaccharides with terminal beta-linked galactose residues. The carbohydrate moiety may be free [lactose or D-galactopyranosyl-beta-(1 leads to 4)-thiogalactopyranoside], protein-bound (asialofetuin) or lipid-bound (cerebrosides). The molecular properties of the beta-galactoside-binding proteins of rat lung and human lung are closely similar to those of embryonic chick muscle lectin [Nowak, Kobiler, Roel & Barondes (1977) Proc. Natl. Acad. Sci. U.S.A. 73, 1383--1387] and calf heart lectin [De Waard, Hickman & Kornfeld (1976) J. Biol. Chem. 251, 7581--7587].

Enzyme-enzyme interaction and the biosynthesis of aromatic amino acids in Escherichia coli

The technique of affinity chromatography has been used to demonstrate that enzymes involved in the biosynthesis of tyrosine and phenylalanine in Escherichia coli undergo reversible interactions. Thus it has been shown that the aromatic amino acid aminotransferase (aromatic-amino-acid: 2-oxoglutarate amino-transferase, EC 2.6.1.57) reacts specifically with chorismate mutaseprephenate dehydrogenase (chorismate pyruvate mutase, EC 5.4.99.5 and prephenate: NAD+ oxidoreductase (decarboxylating), EC 1.3.1.12) in the absence of reactants and with chorimate mutase-prephenatedehydratase (prephenate hydro-lyase (decarboxylating), EC 4.2.1.51) in the presence of phyenylpyruvate. Tyrosine causes dissociation of the aminotransferase: mutasedehydrogenase complex while dissociation of the aminotransferase-mutasedehydratase complex occurs on omission of phenylpyruvate. Only the active form of chorismate mutase-prephenate dehydrogenase participates in complex formation.

Modification of surface composition of Actinomyces viscosus T14V and T14AV

The morphology and serology of Actinomyces viscosus T14V and T14AV were compared. When grown in supplemented tryptic soy broth, the virulent strain (T14V) possessed an extensive network of cell surface fibrils. In this medium, the avirulent strain (T14AV) possessed a microcapsule, absent on strain T14V, and a comparatively small number of surface fibrils. Mild acid extraction (Lancefield procedure) solubilized common antigenic components on both strains as well as components detectable only in the virulent strain T14V (virulence-associated antigens 1 and 2). When grown in Socransky chemically defined medium or Carlsson complex medium, the avirulent strain possessed increased amounts of surface fibrils and virulence-associated antigens. Whole cells and extracts of avirulent cells grown in Socransky medium absorbed antibodies to virulence-associated antigens with approximately the same efficiency as did whole cells and extracts of strain T14V, suggesting antigenic similarity between the two cell types. The results strongly support the hypothesis that observable differences between A. viscosus strains T14V and T14AV represent quantitative, rather than qualitative, differences in particular cell surface components. In addition, the magnitude of these differences can be modified by changing growth conditions.

Role of the Escherichia coli aromatic amino acid aminotransferase in leucine biosynthesis

Strains of Escherichia coli that lack the branched-chain amino acid amino-transferase because of mutations in the ilvE gene had no growth requirement for leucine when the cells contained the aromatic amino acid aminotransferase that is the product of the tyrB gene. The presence of leucine increased the generation time of these cells and decreased the specific activity of the aromatic amino acid aminotransferase. It is concluded that this enzyme functions efficiently in leucine biosynthesis and can be repressed by leucine as well as by tyrosine.

The purification and properties of the aspartate aminotransferase and aromatic-amino-acid aminotransferase from Escherichia coli

A simple and convenient procedure is described for the isolation in good yield of two amino-transferases from various strains of Escherichia coli. On the basis of their substrate specificities one of the enzymes has been classified as an aromatic amino acid aminotransferase and the other as an aspartate aminotransferase, but both act on a wide range of substrates. Pyridoxal phosphate is bound more strongly to the aspartate aminotransferase than to the aromatic amino transferase which cannot be fully re-activated after removal of the prosthetic group. Both enzymes are composed of two subunits which appear to be identical.

Enzymic characteristics of fat globule membranes from bovine colostrum and bovine milk

Fat globule membranes have been isolated from bovine colostrum and bovine milk by the dispersion of the fat in sucrose solutions at 4 degrees C and fractionation by centrifugation through discontinuous sucrose gradients. The morphology and enzymic characteristics of the separated fractions were examined. Fractions comprising a large proportion of the total extracted membrane were thus obtained having high levels of the Golgi marker enzymes UDP-galactose N-acetylglucosamine beta-4-galactosyltransferase and thiamine pyrophosphatase. A membrane-derived form of the galactosyltransferase has been solubilized from fat and purified to homogeneity. This enzyme is larger in molecular weight than previously studied soluble galactosyltransferases, but resembles in size the galactosyltransferase of lactating mammary Golgi membranes. In contrast, when fat globule membranes were prepared by traditional procedures, which involved washing the fat at higher temperatures, before extraction, galactosyltransferase was not present in the membranes, having been released into supernatant fractions, When the enzyme released by this procedure was partially purified and examined by gel filtration, it was found to be of a degraded form resembling in size the soluble galactosyltransferase of milk. The release is therefore attributed to the action of proteolytic enzymes. Our observations contrast with previous biochemical studies which suggested that Golgi membranes do not contribute to the milk fat globule membrane. They are, however, consistent with electron microscope studies of the fat secretion process, which indicate that secretory vesicle membranes, derived from the Golgi apparatus, may provide a large proportion of the fat globule membrane.

Multiple antibiotic resistance in clinical strains of Neisseria gonorrhoeae isolated in South Carolina

Minimal inhibitory concentration (MIC) values for penicillin G, tetracycline, amoxicillin, and actinomycin D were determined for 81 strains of Neisseria gonorrhoeae isolated from patients attending public health clinics in the Piedmont region of South Carolina. Gonococcal isolates were also screened for highlevel resistance to streptomycin. Significant positive correlations (r >/= 0.45, P </= 0.01) were found between all possible pairs of the antibiotics penicillin G, tetracycline, and amoxicillin. The MIC values of actinomycin D showed no significant positive correlations with the MIC values of the other antibiotics. Gonococcal strains that were resistant to streptomycin tended to be resistant to penicillin G, tetracycline, and amoxicillin. Of the 81 isolates, 18.5% were multiply resistant to penicillin G, tetracycline, amoxicillin, and streptomycin. Spontaneous mutants with reduced antibiotic susceptibility, selected for decreased susceptibility to penicillin G, displayed small decreases in susceptibility to tetracycline, amoxicillin, and actinomycin D. Spontaneous mutants selected for decreased susceptibility to actinomycin D displayed small losses in susceptibility to penicillin G. The results show that multiple antibiotic resistance occurs in clinically isolated gonococcal strains in South Carolina. The results further suggest the presence of a common genetic mechanism determining antibiotic resistance in N. gonorrhoeae.

Affinity labeling of bovine colostrum galactosyltransferase with a uridine 5'-diphosphate derivative

The dialdehyde produced by the periodate cleavage of the ribose moiety of uridine 5'-diphosphate (UDP) has been used as an affinity label for the UDP-galactose/UDP binding site of galactosyltransferase from bovine colostrum. This derivative causes progressive inactivation of galactosyltransferase at a rate dependent on its concentration, and under certain conditions is a competitive inhibitor with respect to UDP-galactose. The substrate UDP-galactose protects the enzyme from inactivation. The inactivation is also dependent on Mn2+ concentration in a range that implies that the binding of Mn2+ at site I is a prerequisite for the binding of the UDP derivative. The inactivation can be progressively reversed by nitrogenous bases, or stabilized by KBH4 reduction, which is consistent with the hypothesis that a Schiff base has formed with a lysine residue. Galactosyltransferase was inactivated with a [3H]UDP derivative and the predominant labeled peptide, from thermolysin digestion, isolated and characterized as: Ser-Gly-Lys-UDP.

A comparison of the interactions of galactosyltransferase with a glycoprotein substrate (Ovalbumin) and with alpha-lactalbumin

Sedimentation velocity ultracentrifugation and other procedures have been used to investigate macromolecular interactions of bovine colostrum galactosyltransferase with a glycoprotein substrate (ovalbumin) and with the lactose synthase regulatory protein, alpha-lactalbumin. The determination of equilibrium binding constants for these interactions and the effects of ligands and combinations of ligands on the equilibria have clarified several aspects of the mechanism of galactosyltransferase and its regulation by alpha-lactalbumin. 1. The attachment of Mn2+ at the tight binding site on galactosyltransferase (site I, Kd 2.3 muM) is an essential prerequisite for interactions with ovalbumin and with alpha-lactalbumin. 2. The attachment of Mn2+ or Ca2+ at the weaker metal binding site (site II, Kd 1 to 2 mM) does not significantly affect the interaction of galactosyltransferase with either protein. This is consistent with the hypothesis derived from kinetic studies that site II is functionally connected with the binding of UDP-derivatives. 3. While the binding of ovalbumin to galactosyltransferase in the presence of Mn2+ alone can be observed by ultracentrifugation, this interaction is too weak to cause binding of galactosyltransferase to ovalbumin-Sepharose. Binding to ovalbumin-Sepharose could only be detected by affinity chromatography in the presence of both Mn2+ (10 mM) and UDP (0.3 MM). Sedimentation studies showed that the association of galactosyltransferase with ovalbumin is pressure-dependent, and that the presence of UDP in the complex increases the equilibrium association constant by a factor of 46. The enzyme Mn2+-UDP-ovalbumin complex has unusual hydrodynamic properties. 4. The presence of saturating concentrations of UDP-galactose potentiates the binding of alpha-lactalbumin at high concentrations of Mn2+, as shown by a 25-fold increase in the association constant. Competitive inhibition by alpha-lactalbumin, with respect to ovalbumin that is observed by steady state kinetics, is attributed to the mutally exclusive binding of the proteins with an enzyme complex containing Mn2+ and UDP-galactose. 5. Monsaccharides (N-acetylglucosamine and glucose) strongly enhance the binding of alpha-lactalbumin to enzyme complexes containing Mn2+, in the presence or absence of UDP-glucose. The binding of alpha-lactalbumin and monosaccharide to form enzyme complexes containing both is random, and evaluation of the four associated equilibrium constants shows that the binding is strongly synergistic. 6. Although the significance of some of the many equilibria studied cannot be ascertained, it appears than an element of randomness may be present in reactions catalyzed by galactosyltransferase...

Metal ion activation of galactosyltransferase

Galactosyltransferase, which functions as the catalytic component of lactose synthase and in the glycosylation of glycoproteins, has been previously reported to have an absolute dependence on Mn2+ for activity, with a Kd for Mn2+ (10(-3) M) 2 to 3 orders of magnitude greater than the physiological range of Mn2+ concentrations (v 10(-6) M). Reinvestigation of the metal ion dependence of this enzyme has shown that Zn2+, Cd2+, Fe2+, Co2+, and Pr3+ also produce activation, although with lower activities at saturation than that attained with Mn2+. Velocity against metal ion concentration curves for all metals, including Mn2+, are sigmoid, suggesting the presence of two or more activating metal binding sites on the enzyme. The presence of two sites is confirmed by studies using both Mn2+ and Ca2+. While galactosyltransferase is inactive in the presence of Ca2+ alone, at low concentrations of Mn2+ (10(-5) M), enzyme activity is stimulated by Ca2+. A more detailed investigation by steady state kinetics has revealed that there is a tight binding site for Mn2+ (site I: Kd of 2 X 10(-6) M) from which Ca2+ is excluded, and a site at which Ca2+ can replace Mn2+ (site II: Kd for Ca2+ of 1.76 X 10(-3) M), to which metal binding has a specific synergistic effect on UDP-galactose binding, possibly as a result of the formation of an enzyme-Ca2+-UDP-galactose bridge complex. The site I Mn2+, site II Ca2+-activated enzyme has a maximum velocity similar to that of the Mn2+-activated enzyme, and is the enzyme form that must act in lactose synthesis in vivo. A trypsin-degraded form of galactose transferase (galactosyltransferase-T) (Powell, J.T., and Brew, K. (1974) Eur. J. Biochem. 48, 217-228) appears to lack site I and is activated by Ca2+ in the absence of Mn2+.