Ascorbate increases the synthesis of procollagen hydroxyproline by cultured fibroblasts from chick embryo tendons without activation or prolyl hydroxylase

An improved procedure was developed to extract prolyl hydroxylase from tendon cells of chick embryos with detergent, and improved assays were developed for both the activity of the enzyme and the amount of enzyme protein. Freshly isolated tendon cells were found to contain approx. 100 μg of enzyme protein per 10⁸ cells and 40–50% of the enzyme protein was active. When the cells were cultured, they were found to contain the same amount of enzyme protein by only 15–20% of the enzyme protein was active. Gel filtration of cell extracts indicated that the active form of prolyl hydroxylase in freshly isolated tendon cells and in cultured tendon cells had the same apparent size and the same activity per μg of immunoreactive protein as enzyme which was shown to be a tetramer. The inactive form was found to have about the same apparent size as subunits of the enzyme.When freshly isolated cells were incubated for 2 h in the presence of 40 μg per ml of ascorbate, there was a slight increase in the rate of hydroxyproline synthesis. In cultured cells, ascorbate at a concentration of 40 μg per ml caused a 2-fold increase in the rate of hydroxyproline synthesis within 30 min. However, ascorbate did not increase the activity of prolyl hydroxylase in extracts from either cell system. Therefore it appears that the influence of ascorbate on synthesis of procollagen hydroxyproline by the cells studied here must be ascribed to a cofactor effect on the hydroxylation reaction similar to that observed with purified enzyme, and it does not involve “activation” of inactive enzyme protein to active enzyme as has been observed in cultures of L-929 and 3T6 mouse fibroblasts.     

CRP,immunologically cross-reacting protein of prolyl hydroxylase. Its role in assembly of active prolyl hydroxylase and cellular localization in L-929 fibroblasts

There are two forms of prolyl hydroxylase in L-929 flbroblasts. One is the enzymatically active tetramer having two α- and two β-subunits. The other is monomeric cross-reacting protein which is enzymatically inactive but is structurally related to β-subunit of the enzyme. Cultured L-929 fibroblasts at mid-log phase were labeled by ³H-labeled amino acid mixture and the radioactivity was chased for 24 h while cells were harvested and plated at higher cell densities in cultures. The results indicated that both α-subunit of the tetrameric prolyl hydroxylase and cross-reacting protein were labeled, but the β-subunit of the tetrameric active prolyl hydroxylase was not labeled until the cells were crowded for 24 h. Using immunofluorescent techniques with antibodies directed against pure tetrameric prolyl hydroxylase, capping or patching was observed when the cells were incubated at 37 °C. Also, it was found that phagosomes prepared from L-929 flbroblasts contained about 30% of total enzyme protein as determined immunologically but contained no significant prolyl hydroxylase activities. Labeling cells with ¹²⁵I by lactoperoxidase, cross-reacting protein was labeled but both α- and β-subunits of tetrameric active prolyl hydroxylase were not labeled. The results indicate that cross-reacting protein can be utilized as the precursor of β-subunit by the cells to form tetrameric active prolyl hydroxylase and that cross-reacting protein is found associated with cytoplasmic membranes.     

Ascorbate increases the synthesis of procollagen hydroxyproline by cultured fibroblasts from chick embryo tendons without activation of prolyl hydroxyla.

An improved procedure was developed to extract prolyl hydroxylase from tendon cells of chick embryos with detergent, and improved assays were developed for both the activity of the enzyme and the amount of enzyme protein. Freshly isolated tendon cells were found to contain approx. 100 mug of enzyme protein per 10(8) cells and 40-50% of the enzyme protein was active. When the cells were cultured, they were found to contain the same amount of enzyme protein but only 15-20% of the enzyme protein was active. Gel filtration of cell extracts indicated that the active form of prolyl hydroxylase in freshly isolated tendon cells and incultured tendon cells had the same apparent size and the same activity per mug of immunoreactive protein as enzyme which was shown to be a tetramer. The inactive form was found to have about the same apparent size as subunits of the enzyme. When freshly isolated cells were incubated for 2 h in the presence of 40 mug per ml of ascorbate, there was a slight increase in the rate of hydroxyproline synthesis. In cultured cells, ascorbate at a concentration of 40 mug per ml caused a 2-fold increase in the rate of hydroxyproline synthesis within 30 min. However, ascorbate did not icrease the activity of prolyl hydroxylase in extracts from either cell system. Therefore it appears that the influence of ascorbate on synthesis of procollagen hydroxyproline by the cells studied here must be ascribed to a cofactor effect on the hydroxylation reaction similar to that observed with purified enzyme, and it does not involve "activation" of inactive enzyme protein to active enzyme as has been observed in cultures of L-929 and 3T6 mouse fibroblasts.     

A substance in L-929 cell extracts which replaces the ascorbate requirement for prolyl hydroxylase in a tritium release assay for reducing cofactor; correlation of its concentration with the extent of ascorbate-independent proline hydroxylation and the level of prolyl hydroxylase activity in these cells

Reductant used as cofactor for the prolyl hydroxylase reaction, was measured by a tritium release assay modified from an enzyme assay by making all components of the assay system saturating except for the reductant, but including prolyl hydroxylase. Reduced glutathione (6 mm), which had little activity as a cofactor, and thymol (0.1 mm), an antioxidant which exhibited no cofactor activity at all, were required for optimal proline hydroxylation dependent on reducing cofactor, with thymol fulfilling the previously described requirement for catalase. Ascorbate, cysteine and 6,7-dimethyltetrahydropterin were active as cofactors, in descending order of activity at equimolar concentrations, and activity was concentration dependent for all of these compounds. Sonicates of stationary phase L-929 cells which exhibit ascorbate-independent proline hydroxylation in culture contained reducing cofactor which could replace ascorbate in the cofactor assay, while sonicates of log phase cells which exhibit an ascorbate requirement in culture contained about one-third or less of that amount. NADH and NADPH, which themselves have little or no activity as cofactor, increased the cofactor activity of log phase cell sonicates but had relatively little effect on the activity of stationary cell sonicates suggesting that the cofactor is in a more reduced state in stationary phase. Within 24 h after replating dense, stationary phase cell cultures at low density, conditions where cells return to ascorbate dependence, prolyl hydroxylase activity had decreased to one-fifth the original activity while the concentration of functional reducing cofactor had decreased to less than 1% of its original concentration, largely as a result of oxidation. Ascorbate was not present in L-929 cells sonicates and the levels of tetrahydropterin and cysteine in sonicates could not account for the amount of cofactor activity exhibited by the sonicates in the assay system. Treatment of L-929 cultures with aminopterin did not decrease ascorbate independence, suggesting that tetrahydrofolate did not contribute significantly to cellular proline hydroxylation. These results suggest that an unidentified reductant present in L-929 cells can account for ascorbate-independent proline hydroxylation and also regulate prolyl hydroxylase activity in these cells and that cellular levels of reduced pyridine nucleotides may regulate the reduction state of this substance.     

Cell density-dependent increase in prolyl hydroxylase activity in cultured L-929 cells requires de novo protein synthesis.

Prolyl hydroxylase activity in cultured L-929 cells was found to increase when cells grew from log phase to stationary phase and when cells were harvested at the mid-log phase and replated at higher cell densities. Cycloheximide and actinomycin D inhibited the cell density-dependent increase in prolyl hydroxylase activity indicating that the increase in prolyl hydroxylase activity required de novo synthesis of protein and RNA. Prolyl hydroxylase was purified from cultured L-929 cells and antibodies against the protein were raised in rabbits. The antibodies were used to demonstrate that L-929 cells contained two forms of prolyl hydroxylase: an enzymatically active, tetrameric form consisting of two alpha and two beta polypeptide chains and an enzymatically inactive form containing immunologically cross-reacting protein. The polypeptide chains alpha, beta and cross-reacting protein were obtained by immunoadsorption. Peptide map analysis indicated that cross-reacting protein was similar if not identical to beta in primary structure, and alpha was different from both beta and cross-reacting protein. The results suggested that the prolyl hydroxylase levels in cells or tissues may be regulated by new protein and/or RNA synthesis.     

The association between prolyl hydroxylase metabolism and cell growth in cultured L-929 fibroblasts

Prolyl 4-hydroxylase (EC 1.14.11.2) is a key enzyme in collagen biosynthesis, its active form is a tetramer (alpha 2 beta 2). In L-929 fibroblasts in the log phase of culture there is a low level of active enzyme. When the cell culture reaches confluency, prolyl hydroxylase activity in cells increases by a process that requires de novo RNA and protein synthesis. The same result may be achieved by crowding the cells (replating log phase cells at the density of stationary phase cells). In the work reported here we further examined induction of the enzyme. RNA synthesis necessary for enzyme induction is complete 6 h after "crowding" while protein synthesis requires 12 h. Thymidine (0.2-0.5 mM) added to log phase cells will also cause enzyme induction to the level found in "crowded" or resting cells. We also looked at the decay of the enzyme activity after subculture. This occurs rapidly (enzyme half-life is 1-2 h) and is concurrent with the re-entry of resting cells into cell cycle; however, thymidine added at the time of subculture to block DNA synthesis does not prevent the loss of prolyl hydroxylase activity. These results suggest that when cells are not engaged in propagation, they begin to synthesize luxury proteins such as prolyl hydroxylase. However, the loss of prolyl hydroxylase during subculture is probably not a direct consequence of DNA synthesis.     

The formation of hydroxyproline in collagen by cells grown in the absence of serum.

L-929 and 3T6 cells were conditioned to grow in a chemically defined medium lacking serum and ascorbate. Serum, when added, had a small stimulatory effect on the growth rate of the cells, but ascorbate had no effect either on the growth rate or on the rate of protein synthesis. These cells were also shown to lack gulonolactone oxidase activity and therefore could not synthesize their own ascorbate. Nevertheless, in the absence of serum and ascorbate both cell types were able to hydroxylate peptidyl proline to an appreciable extent. This suggests that reductants other than ascorbate can at least partially satisfy the requirement for a reductant in the prolyl hydroxylase reaction in vivo.     

The formation of hydroxyproline in collagen by cells grown in the absence of serum

L-929 and 3T6 cells were conditioned to grow in a chemically defined medium lacking serum and ascorbate. Serum, when added, had a small stimulatory effect on the growth rate of the cells, but ascorbate had no effect either on the growth rate or on the rate of protein synthesis. These cells were also shown to lack gulonolactone oxidase activity and therefore could not synthesize their own ascorbate. Nevertheless, in the absence of serum and ascorbate both cell types were able to hydroxylate peptidyl proline to an appreciable extent. This suggest that reductant other than ascorbate can at least partially satisfy the requirement for a reductant in the prolyl hydroxylase reaction in vivo.     

An intramembranous reductant which participates in the proline hydroxylation reaction with intracisternal prolyl hydroxylase and unhydroxylated procollagen in isolated microsomes from L-929 cells

We previously have described a substance present in crude sonicates of L-929 cells which replaced ascorbate in vitro as a reductant for prolyl hydroxylase (B. Peterkofsky, D. Kalwinksy and R. Assad, 1980, Arch. Biochem. Biophys.199, 362–373). In the present study we found that almost 90% of the substance was particulate after differential centrifugation of stationary phase L-929 cell homogenates. The substance was not localized in nuclei or mitochondria and was found in the same fractions as microsomes, but these fractions also contained lysosomes and cell membranes. The reductant could not be solubilized from particles by Brij-35, indicating that it is an intrinsic component of a membrane rather than intracisternally located. The intramembranous cofactor, in the absence of ascorbate, participated in the in vitro hydroxylation of [4-³H]proline in radio-actively labeled, intracisternal unhydroxylated procollagen in isolated microsomes which also contained prolyl hydroxylase. Hydroxylation was determined by measuring tritiated water formed from release of the 4-trans tritium atom. Since it is unlikely that such participation could occur if the cofactor were located within the membrane of another subcellular organelle, we have concluded that it is in the same particle as prolyl hydroxylase and unhydroxylated procollagen, that is, the microsome. With the endogenous reductant the reaction was slower than with saturating ascorbate and was increased by NADH. Maximum hydroxylation with the endogenous reductant was close to that which could be achieved with ascorbate. These results provide strong evidence that the endogenous reductant alone can account for the phenomenon of ascorbate-independent proline hydroxylation in L-929 cells. As in the case of ascorbate, the microsomal reductant functioned only in the presence of α-ketoglutarate and Fe²⁺ and served as reductant for lysyl hydroxylase. It also was detected in the particulate fraction of virally transformed BALB 3T3 cells and in purified microsomes from bones of intact chick embryos. Since ascorbate could be taken up and concentrated in bone microsomes, it is unlikely that the endogenous reductant serves as an intermediary between ascorbate and intracisternal prolyl hydroxylase.     

The stimulatory effect of platelet homogenate on prolyl hydroxylase activity in L-929 cells

We have found that the addition of platelet homogenate to confluent cultures of L-929 cells increases 2-3 times the activity of prolyl hydroxylase in these cells. Furthermore, it was found that the platelet homogenate potentiates the effect of ferrous ions and ascorbic acid, which are known activators of prolyl hydroxylase. The effect of the platelet homogenate is diminished by cycloheximide. It seems probable that some products present in the platelet homogenate may promote biosynthesis of the enzyme or they stimulate glycolysis and accumulation of lactic acid, an activator of the hydroxylase.     

Collagen biosynthesis by human skin fibroblasts III. The effects of ascorbic acid on procollagen production and prolyl hydroxylase activity

Human skin fibroblasts were cultured under conditions optimized for collagen synthesis, and the effects of ascorbic acid on procollagen production, proline hydroxylation and the activity of prolyl hydroxylase were examined in cultures. The results indicated that addition of ascorbic acid to confluent monolayer cultures of adult human skin fibroblasts markedly increased tha amount of [³H]hydroxyproline syntehsized. Ascorbic acid, however, did not increase the synthesis of ³H-labeled collagenous polypeptides assayed independently of hydroxylation of proline residues, nor did it affect the amount of prolyl hydroxylase detectable by an in vitro enzyme assay. Also long-term cultures of the cells or initiation of fibroblast cultures in the presence of ascorbic acid did not lead to an apparent selection of a cell population which might be abnormally responsive to ascorbic acid. Thus, ascorbic acid appears to have one primary action on the synthesis of procollagen by cultured human skin fibroblasts: it is necessary for synthesis of hydroxyproline, and consequently for proper triple helix formation and selection of procollagen.     

Regulation of collagen post-translational modification in transformed human and chick-embryo cells.

Changes in the regulation of collagen post-translational modification in transformed cells were studied in three established human sarcoma cell lines and in chick-embryo fibroblasts freshly transformed by Rous sarcoma virus. The collagens synthesized by all but one of these and by all the control human and chick-embryo cell lines were almost exclusively of types I and/or III. The relative rate of collagen synthesis and the amounts of prolyl hydroxylase activity and immunoreactive protein were markedly low in all the transformed human cell lines. The other enzymes studied, lysyl hydroxylase, hydroxylysyl galactosyltransferase and galactosylhydroxylysyl glucosyltransferase, never showed as large a decrease in activity as did prolyl hydroxylase, suggesting a more efficient regulation of the last enzyme than of the three others. The chick-embryo fibroblasts freshly transformed by Rous sarcoma virus differed from the human sarcoma cells in that prolyl hydroxylase activity was distinctly increased, whereas the decreases in immunoreactive prolyl hydroxylase protein and the three other enzyme activities were very similar to those in the simian-virus-40-transformed human fibroblasts. It seems possible that this increased prolyl hydroxylase activity is only a temporary phenomenon occurring shortly after the transformation, and may be followed by a decrease in activity later. The newly synthesized collagens of all the transformed cells that produced almost exclusively collagen types I and/or III had high extents of lysyl hydroxylation, and there was also an increase in the ratio of glycosylated to non-glycosylated hydroxylysine. The data suggest that one critical factor affecting modification is the rate of collagen synthesis, which affects the ratio of enzyme to substrate in the cell.     

Ascorbate-independent proline hydroxylation resulting from viral transformation of Balb 3T3 cells and unaffected by dibutyryl cAMP treatment.

Collagen synthesis, hydroxylation of proline in collagen, and collagen secretion were studied in the contact-inhibited mouse fibroblast line, Balb 3T3; the Kirsten virus transformed line, Ki-3T3; and dibutyryl cAMP (dbcAMP)-treated Ki-3T3 cells, during the various phases of the growth cycle. Transformed cells in both logarithmic and stationary phase produced lower levels of collagen than the parent line but 85-90% of the theoretically possible hydroxyproline residues of the collagen were formed even when ascorbic acid was not added to the culture medium. Moreover, the transformed cells showed only about a 20% increase of collagen secretion upon addition of ascorbate. This was in contrast to the ascorbate requirement for maximal proline hydroxylation and the 2-3 fold stimulation of collagen secretion by ascorbate in the parent Balb 3T3 cells. Although dbcAMP treatment caused Ki-3T3 cells to assume a more normal morphology and increased the relative rate of collagen synthesis to levels similar to that of 3T3, such treatment did not restore an ascorbate requirement for proline hydroxylation or collagen secretion. The specific activity of the enzyme prolyl hydroxylase also was not affected by dbcAMP treatment although collagen synthesis was increased by such treatment. In addition, it was found that ascorbic acid was not effective in activating prolyl hydroxylase derived from Ki-3T3 or dbcAMP-treated Ki-3T3 cell cultures either in logarithmic phase or stationary phase. Ki-3T3 cultures did not accumulate ascorbic acid in cells or medium nor was ascorbic acid synthesized from the precursor 14C-glucuronate in cell homogenates. The results suggest that virally transformed Balb 3T3 cells acquire the capacity to synthesize a reducing cofactor for prolyl hydroxylase and that this function may be related to the increased glycolytic metabolism of these cells since neither cellular metabolism nor ascrobate-independent hydroxylation was altered by treatment with dbcAMP.     

Regulation of prolyl and lysyl hydroxylase activities in cultured human skin fibroblasts by ascorbic acid

Studies with confluent human skin fibroblasts maintained in 0.5% serum supplemented medium have given new insight into the regulatory influences of ascorbate. These include a reduction of prolyl hydroxylase activity, a stimulation of lysyl hydroxylase activity, and an acceleration of collagen production. The lack of parallel between prolyl hydroxylase activity and collagen production indicates that the rate of collagen synthesis is not controlled by the level of prolyl hydroxylase.     

Collagen synthesis by cultured skin fibroblasts from siblings with hydroxylysine-deficient collagen

It has been previously shown that dermis from subjects with hydroxylysine-deficient collagen contains approximately 5% of normal levels of hydroxylysine and sonicates of skin fibroblasts contain less than 15% of normal levels of collagen lysyl hydroxylase activity. However, cultures of dermal fibroblasts from two siblings with hydroxylysine-deficient collagen (Ehlers-Danlos Syndrome Type VI) compared to fibroblasts from normal subjects synthesize collagen containing approximately 50% of normal amounts of hydroxylysine. The lysyl hydroxylase deficient cultures synthesize both Type I and Type III collagen in the same proportion as control cultures. Both α1(I) and α2 chains are similarly reduced in hydroxylysine content. Collagen prolyl hydroxylation by normal and mutant cells is severely depressed without ascorbate but in all cultures collagen lysyl hydroxylation is the same with or without ascorbate supplementation. In mutant cells the rate of prolyl hydroxylation measured after release of inhibition by α,α′-dipyridyl is the same as in control cells. The rate of lysyl hydroxylation is reduced in mutant cells but only to approximately 50% of normal.     

Effect of L-3,4-dehydroproline on collagen synthesis and prolyl hydroxylase activity in mammalian cell cultures.

The incorporation of DL-3,4-dehydro[14C]proline into collagen and total protein of 3T3 cells occurred at approximately one-fifth the rate observed for L-[14C]proline. Addition of L-3,4-dehydroproline to the culture medium inhibited markedly the incorporation of [14C]glycine and L-[3H]lysine into the collagen of 3T3 cells, but there was only slight inhibition of the incorporation of the radiolabeled amino acids into total cellular proteins, indicating that the action of L-3,4-dehydroproline is specific for collagen. When 1 mM L-3,4-dehydroproline was added to the culture medium the [14C]hydroxyproline content was reduced 40% in the cell layer and 70% in the medium. The D isomer of 3,4-dehydroproline did not inhibit [14C]hydroxyproline formation. These findings indicate that L-3,4-dehydroline reduced the hydroxylation of the susceptible prolyl residues in the collagen molecule and the secretion of collagen from the cell. The reduction in the hydroxyproline content is probably related in part to a reduction in the activity of prolyl hydroxylase; when various mammalian cell cultures were exposed to 0.2 mM L-3,4-dehydroproline, the specific activity of prolyl hydroxylase was reduced markedly, while that of lysyl hydroproline, the specific activity of prolyl hydroxylase was reduced markedly, while that of lysyl hydroxylase was not affected. Under these conditions, cell growth and lactic dehydrogenase required protein synthesis. Removal of L-3,4-dehydroproline from the growth medium resulted in a time-dependent increase in the specific activity of prolyl hydroxylase.     

Localization of collagen prolyl hydroxylase to the hepatocyte : Studies in primary monolayer cultures of parenchymal cells from adult rat liver

Significant levels of prolyl hydroxylase activity (prolyl-glycyl-peptide, 2-oxoglutarate: oxygen oxidoreductase; EC 1.14.11.2) have been found in freshly isolated hepatocytes prepared from normal or regenerated adult rat liver and primary non-proliferating monolayer cultures of these cells. Four days after partial hepatectomy, the intact regenerated liver contained two times the normal level of prolyl hydroxylase activity. Freshly isolated hepatocytes contained 24% of the total prolyl hydroxylase activity in normal liver and 47% of that in regenerated liver. Upon incubation of hepatocytes for 24 h in a chemically defined culture medium containing insulin, prolyl hydroxylase activity rose 2- to 3-fold, and gradually declined during the next 48 h. The rise in prolyl hydroxylase activity was blocked by addition of cycloheximide to the culture medium. The presence of prolyl hydroxylase activity in hepatocyte cultures was not likely due to contamination with non-parenchymal liver cells. The latter cells contained less than 20% of the total enzyme activity recovered in all cells isolated from the liver. Furthermore, prolyl hydroxylase was localized by immunofluorescence uniformly to the hepatocytes in culture. Cultured hepatocytes converted [¹⁴C]proline to [¹⁴C]hydroxyproline at rates comparable to those reported for whole liver. However, only a small portion of the hydroxyproline containing product was present as collagen protein, suggesting its rapid degradation in culture. We conclude that the liver parenchymal cell may actively participate in collagen synthesis and possibly in collagen degradation.     

Iron-dependent uptake of ascorbate into isolated microsomes

A preliminary study (J.M. Mata, R. Assad, and B. Peterkofsky (1981) Arch. Biochem. Biophys. 206, 93-104) suggested that chick embryo limb bone microsomes took up and concentrated [14C]ascorbate in the presence of cofactors for prolyl hydroxylase. In the present study, we found that the apparent Km for ascorbate in the hydroxylation of intracisternal unhydroxylated procollagen by endogenous prolyl hydroxylase was approximately an order of magnitude less than the value obtained when enzyme solubilized from microsomes was used with an exogenous substrate. These results are compatible with a concentrative uptake of ascorbate into microsomes. The uptake of [14C]ascorbate into microsomes was confirmed and it required only iron, in either the ferrous or ferric form, and was time and temperature dependent, proportional to microsome concentration, and substrate saturable at 2-3 mM ascorbate. Iron-dependent ascorbate uptake also was observed with L-929 cell microsomes. [14C]Ascorbate seemed to be taken up without prior oxidation, since only unlabeled ascorbate, and not dehydroascorbate, competed for uptake into limb bone microsomes. A functional requirement for Fe2+ in ascorbate transport was demonstrated using the intracisternal proline hydroxylating system. L-929 cell microsomes were preincubated with ascorbate with or without the metal and then external ascorbate was oxidized to inactive dehydroascorbate using ascorbic acid oxidase, which cannot penetrate the microsomal membrane. Samples which did not receive iron during the preincubation received it, along with other requirements for prolyl hydroxylase, in a final incubation to measure hydroxylation. Significant hydroxylation was obtained only in samples incubated with iron prior to oxidase treatment, consistent with the conclusion that an iron-dependent process was required to translocate ascorbate and protect it from the oxidase.     

Primary and secondary effects of ascorbate on procollagen synthesis and protein synthesis by primary cultures of tendon fibroblasts.

The effects of ascorbic acid on collagen biosynthesis were studied in primary cultures of fibroblasts from chick embryo tendons. Addition of ascorbate to the cultures increased the rate of synthesis of procollagen hydroxyproline, but the effect was not explained by activation of prolyl hydroxylase as has been seen in other cell cultures. Instead the increase in the rate of hydroxyproline synthesis appeared to be the result of some direct cofactor effect of the vitamin. In the presence of ascorbate, most of the newly synthesized procollagen was hydroxylated and became triple helical. In the absence of ascorbate, the overall degree of hydroxylation in newly synthesized procollagen was reduced, but a small fraction of newly synthesized procollagen was near-maximally hydroxylated and became triple helical. When cultures were exposed to ascorbic acid for more than 6 h, there was an increase in rate of protein synthesis, rate of procollagen synthesis, and fraction of membrane-bound ribosomes. The increases in these parameters in the presence of ascorbate appeared to be a secondary effect produced by the accumulation of stable triple-helical procollagen in the culture system.     

Collagen synthesis by cultured skin fibroblasts from siblings with hydroxylysine-deficient collagen.

It has been previously shown that dermis from subjects with hydroxylysine-deficient collagen contains approximately 5% of normal levels of hydroxylysine and sonicates of skin fibroblasts contain less than 15% of normal levels of collagen lysyl hydroxylase activity. However, cultures of dermal fibroblasts from two siblings with hydroxylysine-deficient collagen (Ehlers-Danlos Syndrome Type VI) compared to fibroblasts from normal subjects synthesize collagen containing approximately 50% of normal amounts of hydroxylysine. The lysyl hydroxylase deficient cultures synthesize both Type I and Type III collagen in the same proportion as control cultures. Both alpha 1(I) and alpha 2 chains are similarly reduced in hydroxylysine content. Collagen prolyl hydroxylation by normal collagen lysyl hydroxylation is the same with or without ascorbate supplementation. In mutant cells the rate of prolyl hydroxylation measured after release of inhibition by alpha, alpha'-dipyridyl is the same as in control cells. The rate of lysyl hydroxylation is reduced in mutant cells but only to approximately 50% of normal.