Altered vitamin E status in Niemann-Pick type C disease

Vitamin E (α-tocopherol) is the major lipid-soluble antioxidant in many species. Niemann-Pick type C (NPC) disease is a lysosomal storage disorder caused by mutations in the NPC1 or NPC2 gene, which regulates lipid transport through the endocytic pathway. NPC disease is characterized by massive intracellular accumulation of unesterified cholesterol and other lipids in lysosomal vesicles. We examined the roles that NPC1/2 proteins play in the intracellular trafficking of tocopherol. Reduction of NPC1 or NPC2 expression or function in cultured cells caused a marked lysosomal accumulation of vitamin E in cultured cells. In vivo, tocopherol significantly accumulated in murine Npc1-null and Npc2-null livers, Npc2-null cerebella, and Npc1-null cerebral cortices. Plasma tocopherol levels were within the normal range in Npc1-null and Npc2-null mice, and in plasma samples from human NPC patients. The binding affinity of tocopherol to the purified sterol-binding domain of NPC1 and to purified NPC2 was significantly weaker than that of cholesterol (measurements kindly performed by R. Infante, University of Texas Southwestern Medical Center, Dallas, TX). Taken together, our observations indicate that functionality of NPC1/2 proteins is necessary for proper bioavailability of vitamin E and that the NPC pathology might involve tissue-specific perturbations of vitamin E status.

Reduced Smad3 protein expression and altered transforming growth factor-beta1-mediated signaling in cystic fibrosis epithelial cells

Cystic fibrosis (CF) is a disease characterized by an aggressive inflammatory response in the airways. Given the antiinflammatory properties of transforming growth factor (TGF)-beta1, it was our goal to examine components of TGF-beta1-mediated signaling in both a cultured cell model and a mouse model of CF. A CF-related reduction of protein levels of the TGF-beta1 signaling molecule Smad3 was found in both of these model systems, whereas Smad4 levels were unchanged. Functional effects of reduced Smad3 expression are manifest in our cultured cell model, as reduced basal and TGF-beta1-stimulated levels of luciferase expression using the TGF-beta1-responsive reporter construct 3TP-Lux in the CF-phenotype cells compared with control cells. However, TGF-beta1-stimulated responses using the A3-Luc reporter construct were normal in both cell lines. These results suggest that select TGF-beta1-mediated signaling pathways are impaired in CF epithelial cells. This selective loss of Smad3 protein expression in CF epithelium may also influence inflammatory responses. Our data demonstrate that both CF-phenotype cells lacking Smad3 expression, and A549 cells expressing a dominant-negative Smad3, are unable to support TGF-beta1-mediated inhibition of either the interleukin (IL)-8 or the NOS2 promoter. We conclude that a CF-related reduction in Smad3 protein expression selectively alters TGF- beta1-mediated signaling in CF epithelium, potentially contributing to aggressive inflammatory responses.

Examining basal chloride transport using the nasal potential difference response in a murine model

Epithelia of humans and mice with cystic fibrosis are unable to secrete chloride in response to a chloride gradient or to cAMP-elevating agents. Bioelectrical properties measured using the nasal transepithelial potential difference (TEPD) assay are believed to reflect these cystic fibrosis transmembrane conductance regulator (CFTR)-dependent chloride transport defects. Although the response to forskolin is CFTR mediated, the mechanisms responsible for the response to a chloride gradient are unknown. TEPD measurements performed on inbred mice were used to compare the responses to low chloride and forskolin in vivo. Both responses show little correlation between or within inbred strains of mice, suggesting they are mediated through partially distinct mechanisms. In addition, these responses were assayed in the presence of several chloride channel inhibitors, including DIDS, diphenylamine-2-carboxylate, glibenclamide, and 5-nitro-2-(3-phenylpropylamino)-benzoic acid, and a protein kinase A inhibitor, the Rp diastereomer of adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS). The responses to low chloride and forskolin demonstrate significantly different pharmacological profiles to both DIDS and Rp-cAMPS, indicating that channels in addition to CFTR contribute to the low chloride response.

Nitric Oxide Deficiency Contributes to Impairment of Airway Relaxation in Cystic Fibrosis Mice

The pulmonary disease of cystic fibrosis (CF) is characterized by persistent airway obstruction, which has been attributed to chronic endobronchial infection and inflammation. The levels of exhaled nitric oxide (NO) are reduced in CF patients, which could contribute to bronchial obstruction through dysregulated constriction of airway smooth muscle. Because airway epithelium from CF mice has been shown to have reduced expression of inducible NO synthase, we examined airway responsiveness and relaxation in isolated tracheas of CF mice. Airway relaxation as measured by percent relaxation of precontracted tracheal segments to electrical field stimulation (EFS) and substance P, a nonadrenergic, noncholinergic substance, was significantly impaired in CF mice. The airway relaxation in response to prostaglandin E2 was similar in CF and non-CF animals. Treatment with the NO synthase inhibitor NG-nitro-L-arginine methylester reduced tracheal relaxation induced by EFS in wild-type animals but had virtually no effect in the CF mice. Conversely, exogenous NO and L-arginine, a NO substrate, reversed the relaxation defect in CF airway. We conclude that the relative absence of NO compromises airways relaxation in CF, and may contribute to the bronchial obstruction seen in the disease.

In vivo alterations of IFN regulatory factor-1 and PIAS1 protein levels in cystic fibrosis epithelium

Inducible nitric oxide synthase-2 (NOS2) expression has been shown to be reduced in cystic fibrosis (CF) epithelial cells. Reduced NOS2 expression is unexpected, given the inflammatory nature of CF airway disease, and is an indication that cell-signaling mechanisms necessary for proper NOS2 regulation are probably altered in CF epithelium. Therefore, we examined the expression levels of regulatory factors necessary for NOS2 expression in CF epithelium and showed that IFN regulatory factor-1 (IRF-1) is necessary for full NOS2 expression. Mice lacking IRF-1 expression have diminished epithelial NOS2 expression, as well as reduced NO-dependent chloride transport across the nasal epithelia. Furthermore, IRF-1 protein expression is reduced in nasal and intestinal epithelial cells from CF mice, suggesting a possible mechanism for the CF-related reduction of epithelial NOS2 expression. Active signal transducer and activator of transcription-1 (Stat1) is necessary for both NOS2 and IRF-1 expression. We found that protein levels of Stat1 were increased in CF cells, but that the active phosphorylated form of Stat1 was bound to the protein inhibitor of activated Stat1 (PIAS1). We propose that increased levels of PIAS1 diminish certain cell-signaling pathways, resulting in reduced IRF-1 and NOS2 expression in CF epithelial cells.

Cystic fibrosis transmembrane conductance regulator-dependent regulation of epithelial inducible nitric oxide synthase expression

Recent evidence has shown that the inducible form of nitric oxide (NO) synthase (NOS2) has reduced expression in airway epithelia of patients with cystic fibrosis (CF) despite the presence of chronic inflammation. The goal of this paper is to determine whether NOS2 expression is regulated by the presence of functional CF transmembrane conductance regulator (CFTR). Using a human trachea epithelial cell line in which CFTR activity is blocked by the overexpression of the CFTR regulatory domain, we found that loss of CFTR activity reduces NOS2 messenger RNA expression as determined by reverse transcriptase/polymerase chain reaction and reduces overall NO production compared with mock-transfected controls. An in vivo model using mice lacking CFTR expression (cftr -/-), wild-type mice (cftr +/+), and cftr -/- mice that have had human CFTR introduced to the intestinal epithelium using the fatty acid binding protein (FABP) promoter (FABP-hcftr) was also examined. Electrical characterization confirmed that FABP-hcftr mice had corrected electrophysiologic properties compared with cftr -/- mice in the ileum, but FABP-hcftr nasal transepithelial potential difference measurements were identical to cftr -/- values showing specific intestinal correction. NOS2-specific immunostaining revealed that NOS2 expression is evident in sections of ileum and nasal epithelium of cftr +/+ mice but is absent in both tissues in cftr -/- mice. FABP-hcftr mice, however, show strong NOS2 staining in epithelial cells of the ileum but reduced staining in the nasal epithelium, suggesting a CFTR-related influence in the regulation of NOS2 expression in epithelial cells.

DNA polymerase-associated lectin (DPAL) and its binding to the galactose-containing glycoconjugate of the replication complex

The highly purified DNA Pol-alpha from rat prostate tumor (PA-3) and human neuroblastoma (IMR-32) cells appeared to be inhibited by Ricin (RCA-II), and Con-A. Loss of activity (40 to 60%) of a specific form of DNA polymerase from IMR-32 was observed when the cells were treated with tunicamycin [Bhattacharya, P. and Basu, S. (1982) Proc. Natl. Acad. Sci., USA 79:1488-1492]. Binding of ConA and RCA to human recombinant DNA polymerase-alpha showed a specific labile site in the N-terminus [Hsi et al.. (1990) Nucleic Acid Res. 18:6231-6237]. The catalytic polypeptide, DNA polymerase-alpha of eukaryotic origin, was isolated from developing tissues or cultured cells as a family of 180 to 120 kDa polypeptides, perhaps derived from a single primary structure. Immunoblot analysis with a monoclonal antibody (SJK-237-71) indicated that the lower molecular weight polypeptides resulted from either proteolytic cleavage of post-translational modification after specific cleavages. Present results suggest DNA polymerase-alpha from embryonic chicken brain (ECB) contains an alpha-galactose-binding subunit which may be involved in developmental regulation of the enzyme. It was shown before that the catalytic subunit of DNA polymerase-alpha reduces from 186 kDa in 11-day-old ECB to 120 kDa in 19-day-old ECB [Ray, S. et al. Cell Growth and Differentiation 2:567-573] by the treatment with methyl-alpha-galactose. The low molecular weight DNA polymerase activity (120 kDa) can be reconstituted to high molecular weight (Mr = 186 kDa) with an alpha-galactose binding, 56kDa lectin-like protein. Polyclonal antibodies raised against the purified lectin were able to precipitate DNA. Pol-alpha as determined by immunostaining with the polymerase-alpha-specific monoclonal antibody SJK 132-20, suggesting this is a DNA polymerase associated-lectin (DPAL). RCA-II and GS-I-Sepharose 4B chromatographies resulted in significant purification of DNA-alpha and a complete separation of polymerase complex and primase.

Nitric oxide-mediated regulation of transepithelial sodium and chloride transport in murine nasal epithelium

Transepithelial ion transport is regulated by a variety of cellular factors. In light of recent evidence that nitric oxide (NO) production is decreased in cystic fibrosis airways, we examined the role of NO in regulating sodium and chloride transport in murine nasal epithelium. Acute intervention with the inducible NO synthase (iNOS)-selective inhibitor S-methylisothiourea resulted in an increase of amiloride-sensitive sodium absorption observed as a hyperpolarization of nasal transepithelial potential difference. Inhibition of iNOS expression with dexamethasone also hyperpolarized transepithelial potential difference, but only a portion of this increase proved to be amiloride sensitive. Chloride secretion was significantly inhibited in C57BL/6J mice by the addition of both S-methylisothiourea and dexamethasone. Mice lacking iNOS expression [NOS2(-/-)] also had a decreased chloride-secretory response compared with control mice. These data suggest that constitutive NO production likely plays some role in the downregulation of sodium absorption and leads to an increase in transepithelial chloride secretion.

Inducible nitric oxide synthase expression is reduced in cystic fibrosis murine and human airway epithelial cells

It has been reported that exhaled nitric oxide levels are reduced in cystic fibrosis (CF) patients. We have examined the inducible isoform of nitric oxide synthase (iNOS) in the airways by immunostaining and found that iNOS is constitutively expressed in the airway epithelia of non-CF mouse and human tissues but essentially absent in the epithelium of CF airways. We explored potential consequences of lost iNOS expression and found that iNOS inhibition significantly increases mouse nasal trans-epithelial potential difference, and hindered the ability of excised mouse lungs to prevent growth of Pseudomonas aeruginosa. The absence of continuous nitric oxide production in epithelial cells of CF airways may play a role in two CF-associated characteristics: hyperabsorption of sodium and susceptibility to bacterial infections.

Regulation of amiloride-sensitive sodium absorption in murine airway epithelium by C-type natriuretic peptide

We have previously shown that C-type natriuretic peptide (CNP), a guanylate cyclase agonist, can stimulate cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride secretion in murine airway epithelial cells via protein kinase (PK) A activation through the inhibition of cGMP-inhibited phosphodiesterases. In this paper, we show that CNP is also capable of reducing amiloride-sensitive sodium absorption in murine airway epithelium through a cGMP-dependent mechanism that is separate from the CFTR regulatory signaling pathway. Both murine tracheal and nasal tissues exhibit sensitivity to amiloride-sensitive sodium regulation by exogenously added CNP. CNP depolarized the nasal transepithelial potential difference by 6.3 +/- 0.5 mV, whereas the cGMP-inhibited phosphodiesterase inhibitor milrinone actually hyperpolarized the nasal transepithelial potential difference by 2.0 +/- 1.2 mV in mice homozygous for a CFTR stop mutation [CFTR(-/-)]. Inhibition of guanylate cyclase activity and PKG activity in normal mice resulted in an increase in amiloride-sensitive sodium absorption, suggesting that tonic regulation of amiloride-sensitive sodium absorption is in part due to basal cGMP levels and PKG activity.

Type II protein kinase A regulates CFTR in airway, pancreatic, and intestinal cells

The type of protein kinase A (PKA) responsible for cystic fibrosis transmembrane conductance regulator (CFTR) activation was determined with adenosine 3', 5'-cyclic monophosphate analogs capable of selectively activating type I or type II PKA. The type II-selective pair stimulated chloride efflux in airway, pancreatic, and colonic epithelial cells; the type I-selective pair only stimulated a calcium-dependent efflux in airway cells. The type II-selective analogs activated larger increases in CFTR-mediated current than did the type I-selective analogs. Measurement of soluble PKA activity demonstrated similar levels stimulated by type I- and type II-selective analogs, creating an apparent paradox regarding PKA activity and current generated. Also, addition of forskolin after the type I-selective analogs resulted in an increase in current; little increase was seen after the type II-selective analogs. Measurement of insoluble PKA activity stimulated by the analogs resolved this paradox. Type II-selective analogs stimulated three times as much insoluble PKA activity as the type I-selective pair, indicating that differential activation of PKA in cellular compartments is important in CFTR regulation.

In vivo activation of CFTR-dependent chloride transport in murine airway epithelium by CNP

Inhibitors of guanosine 3',5'-cyclic monophosphate (cGMP)-inhibited phosphodiesterases stimulate Cl- transport across the nasal epithelia of cystic fibrosis mice carrying the delta F508 mutation [cystic fibrosis transmembrane conductance regulator (CFTR) (delta F/delta F)], suggesting a role for cGMP in regulation of epithelial ion transport. Here we show that activation of membrane-bound guanylate cyclases by C-type natriuretic peptide (CNP) stimulates hyperpolarization of nasal epithelium in both wild-type and delta F508 CFTR mice in vivo but not in nasal epithelium of mice lacking CFTR [CFTR(-/-)]. With the use of a nasal transepithelial potential difference (TEPD) assay, CNP was found to hyperpolarize lumen negative TEPD by 6.1 +/- 0.6 mV in mice carrying wild-type CFTR. This value is consistent with that obtained with 8-bromoguanosine 3',5'-cyclic monophosphate (6.2 +/- 0.9 mV). A combination of the adenylate cyclase agonist forskolin and CNP demonstrated a synergistic ability to induce Cl- secretion across the nasal epithelium of CFTR(delta F/delta F) mice. No effect on TEPD was seen with this combination when used on CFTR(-/-) mice, implying that the CNP-induced change in TEPD in CFTR(delta F/delta F) mice is CFTR-dependent.

C-type natriuretic peptide increases chloride permeability in normal and cystic fibrosis airway cells

C-type natriuretic peptide (CNP), a hormone which stimulates particulate guanylate cyclase activity, was studied for its ability to stimulate chloride permeability through the cystic fibrosis transmembrane conductance regulator (CFTR) in airway epithelial cells. Two cell lines, Calu-3 and CF-T43, were used as models of normal and cystic fibrosis (CF) airway epithelial cells, respectively. Calu-3 cells, derived from a lung carcinoma, express relatively high levels of wild-type CFTR. CF-T43 is a transformed line derived from a nasal polyp and expresses the mutant CFTR, deltaF508. Calu-3 cells exposed to the nucleotide guanosine-3',5'-monophosphate (cGMP) analogue 8-Br-cGMP exhibit increased 36Cl- efflux, demonstrating that cGMP can mediate changes in chloride permeability. CNP induces a bumetanide-sensitive short circuit current across Calu-3 monolayers. Whole-cell currents stimulated by CNP display linear current-voltage relationships and have inhibitor pharmacology and ion selectivity consistent with CFTR channel activity. Sodium nitroprusside (SNP), an activator of soluble guanylate cyclase, and CNP both increase cGMP levels and short circuit current in Calu-3 cells. In contrast, exposure of CF-T43 cells to CNP resulted in an increased 36Cl- efflux rate only when combined with the adenylate cyclase agonist isoproterenol and the response was sensitive to kinase inhibitors. CF-T43 cells exposed to isoproterenol and SNP showed no increase in chloride efflux. Together, these data indicate that CNP can activate wild-type and mutant CFTR through a cAMP-dependent protein kinase pathway and that the sensitivity of Calu-3 cells for this stimulation is greater than that of the CF-T43 cells.

In vivo activation of the cystic fibrosis transmembrane conductance regulator mutant deltaF508 in murine nasal epithelium

The gene causing cystic fibrosis (CF) encodes the CF transmembrane conductance regulator (CFTR), a cAMP-regulated chloride channel. Mutations in this gene result in reduced transepithelial chloride permeability across tissues affected in CF. Consequently, restoring chloride permeability to these tissues may prove therapeutic. Here we report that a combination of forskolin, an adenylate cyclase activator, and milrinone, an inhibitor of class III phosphodiesterases, increases the magnitude of the potential difference across nasal epithelium of mice homozygous for the most common CF mutation, delta F508, while neither drug alone has a significant effect on potential difference. Transgenic mice lacking CFTR do not respond to the milrinone/forskolin combination, indicating that the effect in delta F508 mice requires CFTR. These results suggest that, by pharmacological means, at least partial CFTR-mediated electrolyte transport can be restored in vivo to CF tissues expressing delta F508.

Activation of endogenous deltaF508 cystic fibrosis transmembrane conductance regulator by phosphodiesterase inhibition

Many heterologously expressed mutants of the cystic fibrosis transmembrane conductance regulator (CFTR) exhibit residual chloride channel activity that can be stimulated by agonists of the adenylate cyclase/protein kinase A pathway. Because of clinical implications for cystic fibrosis of activating mutants in vivo, we are investigating whether deltaF508, the most common disease-associated CFTR mutation, can be activated in airway epithelial cells. We have found that, 36Cl- efflux can be stimulated 19-61% above baseline by beta-adrenoreceptor agonists and cGI-phosphodiesterase inhibitors in transformed nasal polyp (CF-T43) cells homozygous for the deltaF508 mutation. The increase in 36Cl- permeability is diminished by protein kinase A inhibitors and is not mediated by an increase in intracellular calcium concentrations. Preincubation of CF-T43 cells with CFTR anti-sense oligonucleotides prevented an increase in 36Cl- efflux in response to beta-agonist and phosphodiesterase inhibitor. Primary cells isolated from CF nasal polyps gave similar results. These data indicate that endogenous levels of deltaF508 protein can be stimulated to increase 36Cl- permeability in airway epithelial cells.

CFTR-mediated chloride permeability is regulated by type III phosphodiesterases in airway epithelial cells

Chloride channel activity of cystic fibrosis transmembrane conductance regulator (CFTR) requires activation of protein kinase A (PKA) by 3'-5'-cyclic adenosine monophosphate (cAMP). The level of cAMP is controlled by the balance between cAMP synthesis and hydrolysis by adenylate cyclase and phosphodiesterases (PDEs), respectively. CFTR channel activity appears to be most sensitive to the activity of type III cyclic nucleotide PDEs in Calu-3 and 16HBE cells, both derived from airway epithelium and expressing wild-type CFTR. Type III PDEs can be identified by their sensitivity to specific inhibitors such as milrinone and amrinone. In Calu-3 cells, specific inhibition of type III PDEs increased chloride efflux up to 13.7-fold, whereas neither rolipram nor Ro20-1724 (type IV PDE inhibitors) nor 3-isobutyl-1-methylxanthine (IBMX, a nonspecific PDE inhibitor) elicited significant increases. None of these compounds had an appreciable effect on total cellular cAMP levels, yet the effects of milrinone and amrinone on chloride efflux were blocked by treatment of cells with Rp-cAMPS, a cAMP analog that inhibits PKA at the site of cAMP binding. Similarly, H-8, an inhibitor of PKA, reduced milrinone-stimulated chloride efflux, indicating that efflux is mediated through the cAMP/PKA pathway. Whole-cell patch clamp analysis revealed that milrinone generated chloride conductances with properties consistent with those of CFTR. Milrinone elicited chloride currents in a dose-dependent manner and induced CFTR activity in the absence of adenylate cyclase agonists. These data suggest that type III PDEs are specifically involved in CFTR activation in airway epithelial cells and that PDE regulation of CFTR may involve subcellular compartments of cAMP.

Characterization of DNA polymerase-alpha/primase complex from developing embryonic chicken brains

DNA polymerase-alpha and primase activities present in a complex, have been isolated, partially purified, and characterized from embryonic chicken brain. DNA polymerase-alpha activity, characterized by its sensitivity to N-ethyl-maleimide, high sedimentation coefficient (11.3 S), and acidic isoelectric point (5-5.5) was found in all embryonic ages. Primase activity, the enzyme responsible for the initiation of DNA synthesis, co-sedimented with DNA polymerase-alpha activity on a continuous glycerol velocity gradient. A complex containing both DNA polymerase-alpha and primase activities was isolated by DE-23 cellulose column chromatography of cell-free extracts of different embryonic ages of chicken brain. In addition to the primase complexed with DNA polymerase-alpha, a free primase activity was isolated by DE-23 cellulose column chromatography of an ammonium sulfate (0-45%; w/v) precipitated fraction of embryonic chicken brain cell-free extract. DNA polymerase-alpha activity from developing chicken brains in the embryonic stage was purified by immuno-affinity column chromatography. Of all the single-stranded DNA templates tested, primase activity was found to be maximally active with poly dC. Primase activity was not inhibited by a high concentration of alpha-amanitin. The results obtained may provide insight into further understanding of regulation of chromosomal DNA replication in developing tissues.

Inhibition of immunopurified DNA polymerase-alpha from PA-3 prostate tumor cells by platinum (II) antitumor drugs

Cisplatin (cis-diamminedichloroplatinum(II); cis-DDP) is used as an effective drug for treatment of a variety of cancers, such as carcinomas of bladder, ovarian, and testicular origin. Immunopurified DNA polymerase-alpha from rat prostate tumor PA-3 cells was inhibited (50%) in the presence of cis-DDP (165 microM) and PtCl2(en) (cis-dichloroethylenediamine platinum (II); DEDAP) (75 microM) and remained uninhibited in the presence of trans-DDP. Immunopurified DNA polymerase-alpha was preincubated with cis-DDP and separated from unreacted cis-DDP by gel filtration chromatography. The platinated DNA polymerase-alpha was unable to initiate the DNA chain extension reaction. N-ethylmaleimide (1 mM), a thiol group modifier, also inhibited (95%) the DNA polymerase-alpha catalyzed reaction in vitro. Possible disruption of a zinc-finger motif of the DNA polymerase-alpha polypeptide chain by replacement of zinc is suggested.

Developmental expression of the embryonic chicken brain DNA polymerase alpha and its binding with monoclonal antibodies against human KB cell DNA polymerase alpha

Changes in DNA polymerase alpha activity accompanying tissue development have been well established in several systems. In most cases, DNA polymerase alpha activity decreases with development. Here, we report observed changes in DNA polymerase alpha activity throughout embryonic chicken brain (ECB) development. The level of DNA polymerase alpha activity was found to gradually decrease by 60% (2.3 to 0.8 nmol of [3H]dCMP incorporated/mg protein/h) between 9- and 19-day-old ECB. An enzyme-linked immunosorbent assay of DNA polymerase alpha utilizing monoclonal antibody SJK 237-71 (human KB cell DNA pol-alpha binder) also demonstrated a gradual decrease (up to 60%) of antigen over this same range of development. Analysis of DNA polymerase alpha from 11- and 19-day-old ECB by a 10 to 30% glycerol density gradient revealed a high molecular weight peak sedimenting near catalase (11.3 S) with activity at the 11th day being approximately 3-fold greater than activity at the 19th day. A Western immunoblot analysis utilizing monoclonal antibody SJK 237-71 (against human KB cell DNA polymerase alpha) showed a decrease in DNA polymerase alpha from 186 kilodaltons in 9- and 11-day ECB cell-free extracts to 120 kilodaltons in extracts from 13- to 19-day ECB. The conversion of DNA polymerase alpha from a higher to a lower molecular weight form may be a regulatory mechanism in eukaryotic DNA replication.