Effect of proline analogs on oxygen toxicity-induced pulmonary fibrosis in the rat

Proline analogs inhibit collagen biosynthesis and prevent accumulation of collagen in tissues. The antifibrotic effects of three proline analogs, cis-hydroxyproline, L-azetidine-2-carboxylic acid, and L-3,4-dehydroproline, were compared in a rat oxygen toxicity model. The specificity of these agents for collagen was examined by measuring their effects on noncollagen protein and elastin accumulation in the lung. Increased lung collagen was produced by exposing rats to 95% O2 for 60 hr followed by a 2-week recovery period. Animals were treated with the proline analogs for the 2-week period. Oxygen exposure in untreated animals increased lung collagen 26% above air-breathing controls, and this increase was prevented by all three analogs. Increased noncollagen protein was also prevented by these agents, suggesting they were not entirely specific for collagen. Elastin accumulation, however, was not inhibited by cis-hydroxyproline. It was concluded that proline analogs were antifibrotic, but affected the metabolism of noncollagen protein.

Effects of glycoprotein and basement membrane synthesis inhibitors on the growth of cultured renal collecting duct epithelium

The biochemical and morphological extent of glycoprotein synthesis inhibition of cellular and extracellular proteins was studied on cultured renal collecting duct (CD) epithelium. We found that tunicamycin (4 micrograms/ml) inhibits the glycosylation of a 150,000 d glycoprotein (gpCDI). A 85,000 d glycoprotein (gpCDII) was not affected. The inhibition by tunicamycin demonstrates that gpCDI has characteristics of a N-glycan, whereas gpCDII seems to be an O-glycan. 6-diazo-5-oxo-norleucine (4 X 10(-5) M) which was used as glutamine analogue, did not show a comparable inhibitory effect as seen with tunicamycin. The lack of effect of norleucine demonstrates that glutamine is not the locus of glycosylation in both proteins. However, because of the tunicamycin inhibition it points to asparagine as the site of glycosylation in the gpCDI. Long term cultures of the tissue up to 15 days in the presence of tunicamycin and norleucine and of substances usually used as basement membrane inhibitors, such as hydroxy-D-proline (1 mM), L-azetidine-2-carboxylic acid (1 mM) and o- and p-nitrophenyl-xylopyranoside 1 mM), revealed that it is possible to eliminate completely the fibroblasts beneath the cultured epithelium and within the degenerating corematerial. Experiments with hydroxy-D-proline showed the most striking effect. Experiments with L-azetidine-2-carboxylic acid and nitrophenyl-xylopyranoside resulted in the elimination of fibroblasts and dedifferentiation of the collecting duct epithelium.

Comparative radioautographic study of the effects of L-azetidine-2-carboxylic acid on matrix secretion and Golgi of the mouse incisor

The effect of a proline analog, L-azetidine-2-carboxylic acid (LACA), on protein matrix secretion by odontoblasts and ameloblasts was compared by light and electron microscopic radioautography after injection of 3H-glycine in young mice. LACA inhibited the secretion of dentin matrix with consequent accumulation of 3H-glycine labeled procollagen in the cisternae of the rough endoplasmic reticulum. In contrast, LACA had no apparent effect on ameloblasts as enamel matrix continued to be packaged in the Golgi apparatus and secreted from Tomes' process within 30 min after injection of the radioprecursor. Electron microscopy revealed that LACA did not cause any change in ameloblast ultrastructure but produced a marked alteration of the odontoblast Golgi complex. All odontoblast Golgi saccules and collagen secretion granules disappeared within 2 h after LACA administration. Odontoblast Golgi cisternae, however, appeared not to be affected. These observations confirm previous studies conducted in this laboratory showing that Golgi saccules in collagen-secreting cells are the initial staging areas for the formation of secretory granules. These results also indicate that a close correlation exists between form and function in the Golgi apparatus of collagen-secreting cells.

[Incorporation of L-azetidine-2-carboxylic acid into collagen of the skin. Structural changes]

As previously shown by two dimensional thin-layer chromatography L-azetidine-2-carboxylic-acid (L-Az) is incorporated into type I skin collagen instead of proline when 3 week old mice are fed with a 0,1% solution of L-Az orally. Ultrastructural investigations did not reveal significant changes in collagen periodicity and on fibril diameter. The collagen fibrils of the upper papillary dermis seemed to be packed more densely, sometimes only one electron dense lamina was seen instead of basal lamina and plasma membrane. The glycosaminoglycane-induced fibrillogenesis was not changed in contrary to the collagen-heat-gelation fibrillogenesis at 37 degrees C, where no gel aggregation could be seen. The reconstruction of native fibres from collagen solutions was disturbed too, several finer precipitated fibrils being detectable. On infrared spectroscopy significant differences in absorption spectra were detected. Correlating with previous results of reduced tensile strength and normal melting point of L-Az collagen we can conclude that L-Az might cause rather intermolecular than intramolecular disturbances of crosslinking.

Collagen metabolism and spicule formation in sea urchin micromeres

The role of collagen or collagen-like protein(s) in the in vitro formation of the sea urchin embryonic skeleton was investigated using isolated micromeres of Strongylocentrotus purpuratus. Micromeres were cultured in sea water containing 4% horse serum on tissue culture plastic or an extracellular matrix of type I collagen. The effect of proline analogs and an inhibitor of collagen hydroxylation on in vitro spicule formation in both culture systems was monitored. When micromeres are cultured in the presence of proline analogs L-azetidine-2-carboxylic acid and L-3, 4-dehydroproline which disrupt collagen metabolism, spicule formation is significantly less inhibited on a collagen substratum than on plastic. Culturing micromeres on plastic in the presence of alpha, alpha'-dipyridyl, an inhibitor of collagen hydroxylation, resulted in almost complete inhibition of spicule formation. The inhibition by alpha, alpha'-dipyridyl can be overcome by culturing micromeres on collagen substratum. These results do not support the idea of collagen being the calcified organic matrix of the spicule. Rather, they suggest that micromeres synthesize a collagen-like extracellular matrix which is necessary for spicule formation. Inhibition of this activity by proline analogs or a collagen processing inhibitor can be overcome by providing the cells with a previously deposited extracellular matrix.

Mutant cell lines resistant to azetidine-2-carboxylic acid: alterations in the synthesis of proline from glutamic acid

Two mutant Chinese hamster lung fibroblast lines have been isolated that are resistant to the toxic proline analog L-azetidine-2-carboxylic acid. The line designated AZCA-1 has 30-fold elevated activity of pyrroline-5-carboxylate synthase and a large increase in the rate of proline production and release compared to controls. Pyrroline-5-carboxylate synthase activity is not elevated in the resistant line designated AZCA-4, but the enzyme is less sensitive to inhibition by ornithine and proline than control enzyme. Intracellular proline is elevated in AZCA-4 cells, with no change in the rate of release of proline synthesized from glutamate. Resistance to azetidine carboxylic acid in both mutant lines is attributed to the expanded intracellular proline pool that results from alterations in pyrroline-5-carboxylate synthase. These results indicate that intracellular proline levels are determined at least in part by the regulated activity of pyrroline-5-carboxylate synthase.

Cotransport of proline and Li+ in Escherichia coli

Uptake of Li+ induced by the addition of proline to a cell suspension of Escherichia coli was detected using an Li+-selective electrode. This Li+ uptake was inhibited by L-azetidine 2-carboxylic acid, a competitive inhibitor of the proline transport system. Thus, direct evidence for Li+-proline cotransport via the proline transport system was obtained. Kinetic parameters of the Li+ uptake were determined.

Tissue interactions in the organization and maintenance of the muscle pattern in the chick limb

Recent investigations on a hereditary muscular dysgenesis (cn/cn) in the chicken (Kieny, Mauger, Hedayat & Goetinck, 1983) have suggested that limb muscle pattern development and subsequent maintenance are two independent steps in the formation of the musculature. The respective activities or muscle cells and connective tissue cells in the ontogeny of the musculature have been investigated in avian embryos 1) by in ovo administration of drugs interfering with collagen biosynthesis, and 2) by heterogenetic somite-exchange experiments between normal and mutant embryos. None of the drugs administered to the chick embryo caused any disturbance of muscle pattern formation or maintenance whether treatment occurred before (5 days) or after (7.5 days) the muscle splitting period. Heterogenetic implantations were performed at 2 days of incubation either at the leg or at the wing level. Somitic mesoderm from non-mutant quail embryo was grafted to replace a piece of somitic mesoderm in putative mutant (cn/cn) chick embryos. The introduction of normal myogenic cells into a mutant leg or wing led to a normally patterned musculature, which demonstrates that the muscular dysgenesis cn/cn results from a defect of the somitic myogenic cell line.

Epithelial induction of osteogenesis in embryonic chick mandibular mesenchyme: a possible role for basal lamina

The initiation of osteogenesis at 7 days in the embryonic chick mandibular mesenchyme depends on an epithelial induction in the mandible to day 4. This article reviews a series of experiments conducted to study the nature of this inductive mechanism. Transfilter tissue recombinations were used to determine whether direct tissue apposition was required for induction. Ultrastructural studies of the epithelial-mesenchymal interface were conducted to see if direct epithelial-mesenchymal cell-cell contacts occurred during the inductive stage in vivo. Epithelial cells were cultured on Millipore filters for 28 days and allowed to deposit extracellular products. These products were tested for inductive activity. Findings from these three sets of experiments were discussed with respect to the inductive mechanism. Our results indicate that the induction is not mediated by a diffusible substance and that direct apposition of the two tissues is required. The mechanism of induction, however, does not require direct epithelial-mesenchymal cell to cell contacts. This suggests that a nondiffusible component of the extracellular matrix may be involved. Epithelial extracellular products are inductively active and have the appearance of basal lamina. The active component of the extracellular product is proteinaceous, perhaps collagen, and appears to be situated in the epithelial basal lamina. The role of basal lamina in epithelial-mesenchymal interactions is discussed.

Proline analogues inhibit human skin fibroblast growth and collagen production in culture

Several structural analogues of proline have been shown to be incorporated into proteins in place of proline. As a consequence, the proliferation of cells in culture and the extracellular deposition of collagen in animal systems are reduced. In this study, the effects of two proline analogues, cis-4-hydroxy-L-proline and L-azetidine-2-carboxylic acid, on the growth parameters and procollagen production by cultured normal human skin fibroblasts were examined. The results indicated that incubation of the cells with the analogues reduced the rate of fibroblast proliferation and lowered the plating efficiency. Further experiments demonstrated that fibroblasts in the presence of L-azetidine-2-carboxylic acid synthesized procollagen polypeptides which were not in a triple-helical conformation, as judged by limited pepsin proteolysis. Also, a significantly increased fraction of the newly synthesized collagenous peptides was in a dialyzable form, suggesting increased degradation of the nonhelical chains. The rate of translation of collagenous polypeptides and the preprocollagen messenger RNA activity in the cells were not affected by the analogues. The proline analogues thus appear to inhibit the production of procollagen on the posttranslational level by preventing the polypeptides from folding into a stable triple-helical conformation. The nonhelical polypeptides are then readily susceptible to proteolysis leading to reduced deposition of extracellular collagen fibers. Similar experiments were also performed with fibroblasts cultured from patients with active progressive systemic sclerosis. Quantitatively and qualitatively comparable inhibition of procollagen production by L-azetidine-2-carboxylic acid was noted with scleroderma cells as with control fibroblast cultures. The results suggest, therefore, that proline analogues may, in the future, prove useful in limiting excessive collagen deposition in scleroderma and other forms of dermal fibrosis.

[Incorporation of L-acetidine-2-carboxylic acid in type I skin collagen. Biochemical and mechanoelastic properties]

In order to elucidate the role of L-acetidine-2-carboxylic acid (L-ac) incorporated into collagen type I of the skin instead of proline, we looked for the mechanoelastic properties of skin. Mice were orally fed with 0, 1% solution of L-ac for 5 weeks, then sacrificed, and type I collagen was extracted. Incorporation of the proline analogue (L-ac) could be shown by two dimensional thinlayer chromatography. The melting point (Tm) of type I collagen was determined by circular dichroism: 36, 5 +/- 1 degrees C for normal collagen, 37 +/- 1 degrees C for L-ac-collagen. This insignificant difference indicates that there was no alteration of the thermal stability of the collagen helix after incorporation of L-ac. Tensile strength was examined on whole strips of skin and worked out at means = 1,00 N/mm, s = 0,20 N/mm, sigma = 0,04 N/mm for normal individuals and means = 0,62 N/mm, s = 0,24 N/mm, sigma = 0,05 N/mm for L-ac-fed animals. The considerable difference could be estimated by the Wilcoxon test (p less than 0,005). As the stability of the collagen helix (shown through melting point determination) has not decreased, the reduced mechanoelastic property of tensile strength seems to be due to intermolecular rather than to intramolecular disturbances of the cross connection of the triple helix.

Production of amino acids by analog-resistant mutants of the cyanobacterium Spirulina platensis

Mutants of Spirulina platensis resistant to 5-fluorotryptophan, beta-3-thienyl-alanine, ethionine, p-fluorophenylalanine, or azetidine-2-carboxylic acid were isolated. Some of these mutants appeared to be resistant to more than one analog and to overproduce the corresponding amino acids. A second group was composed of mutants that were resistant to one analog only. Of the latter mutants, one resistant to azetidine-2-carboxylic acid was found to overproduce proline only, whereas one resistant to fluorotryptophan and one resistant to ethionine did not overproduce any of the tested amino acids.

Effect of prevention of procollagen triple-helix formation on proline 3-hydroxylation in freshly isolated chick-embryo tendon cells

Inhibition of procollagen triple-helix formation by the addition of cis-hydroxyproline or azetidine-2-carboxylic acid increased the synthesis of 3-hydroxy[14C]proline 1.7-1.8-fold in pulse-chase experiments with freshly isolated chick-embryo tendon cells. The amount of 3-hydroxy[14C]proline, expressed as a percentage of the total 14C radioactivity in hydroxyproline, reached 8.4%. Control experiments indicated that the two analogues had no effect on the prolyl 3-hydroxylase activity of these cells. The data suggest that the time available before triple-helix formation in part regulates the extent of the 3-hydroxylation of proline in the biosynthesis of collagen in intact cells.

The role of epithelial collagen and proteoglycan in the initiation of osteogenesis by avian neural crest cells

Osteogenesis was inhibited when mandibular processes from 3 1/2-day-old embryos were cultured in BUdR, LACA, alpha, alpha'-Dipyridyl, 4-Methylumbelliferone, and 4-Methylumbelliferyl-beta-D-glucoside or beta-D-xyloside. Mandibular processes were then cultured in the test substances for 3 days, enzymatically separated into their epithelial and ectomesenchymal components, combined with mandibular components from untreated embros, and either organ-cultured or grafted to chorioallantoic membranes of host embryos. Osteogenesis was inhibited when treated epithelium, but not when treated ectomesenchyme, was present in the tissue recombinations. Analysis of the known action of these inhibitors indicates that proliferation, hydroxylation of collagen, and synthesis of proteoglycans by epithelial cells are all necessary components of this osteogenic epithelial-ectomesenchymal interaction.

Phosphorylation of a ribosomal protein and of virus-specific proteins in cells infected with herpes simplex virus

In cells infected with herpes simplex virus a protein associated with the small subunit of ribosomes became phosphorylated. It was not detectably labelled with 14C-amino acids added after infection and is therefore probably a cellular protein. The phosphorylated ribosomal proteins from HSV-1- and HSV-2-infected cells were indistinguishable electrophoretically and had an apparent mol. wt. of about 48 000. Phosphorylation of the 48K protein was detected 2 to 3 h after infection and reached a maximum rate at 4 to 5 h. It was prevented by adding cycloheximide at 2 h, or actinomycin at 1.5 h p.i., or azetidine at the beginning of infection. The phosphorylation did not occur on reversal of a cycloheximide block in the presence of actinomycin, confirming that it is not caused by a virus alpha-polypeptide. Virus that had been irradiated with u.v. light, although still able to suppress synthesis of cellular protein and DNA, did not induce phosphorylation of the 48K ribosomal protein. Therefore the phosphorylation is not responsible for the suppression of host synthesis. The alpha polypeptides ICP 4, 0, 22 and 27 are also phosphorylated but, in contrast to that of the ribosomal protein, their phosphorylation does not depend on the synthesis of beta and gamma polypeptides. It is probably mediated by a host enzyme.

L-Proline and related compounds: correlation of structure, amnesic potency and anti-spreading depression potency

The effects of L-proline, D-proline, and L-azetidine-2-carboxylic acid (L-A.2.C., the lower homolog of L-proline) have been compared in two systems. L-Proline is more potent than either analog in causing amnesia of one-trial avoidance conditioning of the 2-day-old chick and in preventing mechanically induced spreading depression in the retina isolated from 2-3-week-old chicks. The results suggest that the L-configuration and the proper molecular size are essential for the effects of L-proline upon memory and upon spreading depression. This level of specificity is greater than that involved in protein synthesis because L-A.2.C. is incorporated into protein in place of L-proline, in several protein-synthesizing systems.

Effects of amino acid analogues on protein synthesis and degradation in isolated cells

1. Naturally-occurring and synthetic analogues of phenylalanine, tyrosine, histidine, arginine, proline, tryptophan and the sulphur amino acids have beeen tested in rat reticulocytes and in the Reuber H35 hepatoma for effects on protein synthesis and protein degradation and on the heat lability of phosphoenolpyruvate carboxykinase (EC 4.1.1.32) in the hepatoma cells. The experiments were designed to test whether the analogues could be incorporated into mammalian proteins and whether the resultant proteins would be degraded at an accelerated rate. 2. Several analogues, including thiazolylanine, triazolalanine and selenocystine both stimulated protein synthesis and produced labile protein in reticulocytes. Other analogues, such as dihydroxyphenylalanine, thioproline and pipecolic acid accelerated protein breakdown but probably indirectly via an inhibition of protein synthesis. Azetidine-2-carboxylic acid had the largest effect on protein breakdown in reticulocytes. 3. Labile protein was produced in hepatoma cells incubated in the presence of azetidine-2-carboxylic acid, canavanine, indospicine, triazolalanine, 2-, 3- and 4-fluorophenylalanine. These same analogues, together with 3,4-dehydroproline, beta-2-thienylalanine, dihydroxyphenylalanine, histidinol, 5- and 6-fluorotryptophan, selenocystine and selenomethionine produced heat-labile phosphoenolpyruvate carboxykinase. Enzyme induced in the presence of selenomethionine or indospicine showed the largest increases in heat lability, and for these analogues equimolar concentrations of methionine and arginine respectively were needed to nullify the enzyme abnormality. 4. The toxicity of the same naturally-occurring analogues has been discussed in terms of their ability to be incorporated into cell proteins.

Use of the L-proline analog, L-azetidine-2-carboxylic acid (LACA) to analyse embryonic growth and determination and expression of the chondrogenic phenotype in vivo and in vitro

The L-proline analog, L-azetidine-2-carboxylic acid, (LACA) was injected into embryonated eggs of the common fowl, Gallus domesticus at daily doses of 350 microgram/egg on one or several days betweeh 8 and 12 days of incubation. Treatment at nine-days of incubation preferentially retarded embryonic growth to the twelfth day but recovery of growth rate occurred by 15 days of incubation. Relationships between growth and LACA-inhibited aspects of collagenogenesis are discussed. The earliest aged embryos from which isolated stem cells from membrane bones will form secondary cartilage is ten days of incubation. Secondary chondrogenesis on the quadratojugal, a membrane bone of the skull, was inhibited by treatment of whole embryos with LACA at nine days of incubation but not by treatment at eight days. We concluded that an event involving collagen began at nine days of incubation, was blocked by LACA and was part of the process of chondrogenic determination of these stem cells. Addition of LACA to the medium in which already determined stem cells from the quadratojugal were cultured prevented expression of the chondrogenic phenotype. This proline analog is then a useful probe for events relating both to determination and to expression of the differentiated state, and allows conclusions to be drawn regarding the role of collagenogenesis in these events.

Interference with thyroid histogenesis by inhibitors of collagen synthesis

Histogenesis of thyroid follicles in the chick embryo begins with a penetration by cells of the mesenchymal capsule into a solid epithelial primordium. Before penetration occurs, slits containing fibrillar material form between the epithelial cells. The fibrillar material is an epithelial cell product as shown by its formation within channels that form in cultures of isolated epithelial primordia. The drugs L-azetidine-2-carboxylic acid (LACA) and alpha, alpha'-dipyridyl, which interfere with collagen synthesis, prevent the formation of fibrils in cultured epithelial primordia and in cultures of whole thyroids. Furthermore, mesenchymal cells do not invade when whole thyroid primordia are cultured in the presence of either drug. The effects of alpha, alpha'-dipyridyl are reversed by washing out the drug; the effects of LACA are reversed by incubation with equimolar or greater amounts of L-proline added to the medium along with the drug. The results are interpreted to mean that the fibrillar material is collagen of epithelial origin, that the collagen in some way plays a role in mesenchymal penetration of the epithelial primordium, and that the epithelium is responsible for the pattern of lobulation within the developing gland.

Variant Chinese hamster cells resistant to the proline analog L-azetidine 2-carboxylic acid

Variants resistant to the toxic effects of the proline analog L-azetidine 2-carboxylic acid (AZCA) have been isolated from the Chinese hamster tissue culture line G3 by a three-step selection procedure using increasing concentrations of AZCA. Cells surviving each of the three selective steps have been examined for AZCA resistance and for proline uptake, biosynthesis, and degradation. The largest increment in AZCA resistance is acquired in the third step and is due to overproduction of proline as a result of increased activity of the enzyme system responsible for the conversion of glutamic acid to glutamic gamma-semialdehyde. It is not accompanied by an increase in the rate of formation of proline from ornithine or in the rate of proline uptake or degradation.

Analysis of intracellular feline leukemia virus proteins II. Generation of feline leukemia virus structural proteins from precursor polypeptides

The synthesis and processing of feline leukemia virus (FeLV) polypeptides were studied in a chronically infected feline thymus tumor cell line, F-422, which produces the Rickard strain of FeLV. Immune precipitation with antiserum to FeLV p30 and subsequent sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) were used to isolate intracellular FeLV p30 and possible precursor polypeptides. SDS-PAGE of immune precipitates from cells pulse-labeled for 2.5 min with [35S]methionin revealed the presence of a 60,000-dalton precursor polypeptide (Pp60) as well as a 30,000-dalton polypeptide. When cells were grown in the presence of the proline analogue L-azetidine-2-carboxylic acid, a 70,000-dalton precursor polypeptide (Pp70) was found in addition to Pp60 after a 2.5-min pulse. The cleavage of Pp60 could be partially inhibited by the general protease inhibitor phenyl methyl sulfonyl fluoride (PMSF). This partial inhibition was found to occur only if PMSF was present during pulse-labeling. Intracellular Pp70 and Pp60 and FeLV virion p70, p30, p15, p11, and p10 were subjected to tryptic peptide analysis. The results of this tryptic peptide analysis demonstrated that intracellular Pp70 and virion p70 were identical and that both contained the tryptic peptides of FeLV p30, p15, p11, and p10. Pp60 contained the tryptic peptides of FeLV P30, P15, and P10, but lacked the tryptic peptides of P11. The results of pactamycin gene ordering experiments indicated that the small structural proteins of FeLV are ordered p11-p15-p10-p30. The data indicate that the small structural proteins of FeLV are synthesized as part of a 70,000-dalton precursor. A cleavage scheme for the generation of FeLV p70, p30, p15, p11, and p10 from precursor polypeptides is proposed.

[Biosynthesis and properties of collagen forming in the cartilage of chick embryos in the presence of various proline analogs]

Effect of proline analogues on biosynthesis and properties of collagen in chick embryo cartilage was studied. Azethidine-2-carboxylic acid, 3,4-dehydroproline, cis-fluoro-proline, cis-hydroxyproline and thiazolidine-4-carboxylic acid were found to inhibit the incorporation of 14C-proline into proteins by 66-89%. Introduction of proline analogues, instead of the amino acid, into the polypeptide chains of collagen caused the impairment of the enzymatic hydroxylation of the proline residues (formation of hydroxyproline). 3,4-dehydroproline possessed the highest inhibitory effect (80%) on formation of hydroxyproline. When it was introduced into alpha-chains of protocollagen, 3,4-dehydroproline inhibited 1.5-6-fold the cleavage of the protein by bacterial collagenase.

In vitro utilization of exogenous procollagen by embryonic tooth germs

Embryonic mouse tooth germs treated with L-azetidine-2-carboxylic acid cease their development, undergo a regression of the enamel organ, and do not maintain the histological characteristics of the explanted dental organ. On the other hand if procollagen is added exogenously to explants continously treated with L-azetidine, the effects of the inhibitor are not seen and the tissue is maintained. Thus, exogenously supplied procollagen supports morphogenesis in tooth rudiments that are unable to synthesize procollagen.

Evidence contrary to the protein error hypothesis for in vitro senescence

A strain of diploid fibroblasts, obtained from the skin of a male infant, was cultured in vitro and cells were tested throughout their lifespan for the appearance of altered glucose-6-phosphate dehydrogenase (G-6-PD) detected either by thermostability studies or by immunotitration. No significant difference was found in the proportion of thermolabile enzyme in 31 young cultures (4.8 +/- 1%, S.E.), in comparison with that in 19 old cultures (4.9 +/- 1%, S.E.). Old cultures had ceased active cell division (49-60 doublings); DNA replication, measured by [3H]thymidine uptake over a period of 24 hours, was limited to less than 5% of these cells. Young cells (5-22 doublings) had a [3H]thymidine labeling index of 75-85%. Titration of G-6-PD activity in extracts of young and old cells with neutralizing antibody directes specifically against G-6-PD failed to detect an increment of enzymatically defective G-6-PD in old cells. The thermostability studies were capable of detecting altered G-6-PD in skin fibroblasts from a female heterozygous for a thermolabile mutant of G-6-PD, and in fibroblasts treated with a proline analogue, azetidine carboxylic acid. The immunotitration technique was also capable of detecting catalytically altered G-6-PD from the thermolabile mutant and G-6-PD inactivated with N-ethylameimide. These findings argue against a protein error catastrophe as the cause of in vitro clonal senescence.

Effect of L-azetidine 2-carboxylic acid on growth and proline metabolism in Escherichia coli

The effects of L-azetidine 2-carboxylic acid on growth and proline metabolism in a proline-requiring auxotroph of Escherichia coli are described. The homologue inhibited growth of the wild type and it, alone, did not substitute effectively for proline as a growth supplement for the mutant. In medium containing 0.05 mM proline, the addition of increasing amounts of homologue progressively inhibited growth of the wild type but stimulated growth of the mutant at homologue: proline ratios of 10 : 1 and 50 : 1. This suggested that the homologue exerted a "sparing effect" on proline in the mutant. The incorporation of L-[U-14C]proline and L-[3H]azetidine 2-carboxylic acid into hot trichloroacetic acid-insoluble material in the mutant was measured. Amino acid analysis of the insoluble material from cells incubated with radiolabeled proline alone revealed that proline was partially degraded and metabolized to other amino acids prior to incorporation into protein. The addition of unlabeled homologue to the incubation medium significantly reduced proline catabolism, suggesting that the homologue exerted a sparing effect on proline in this mutant. In medium containing unlabeled proline and radiolabeled L-azetidine 2-carboxylic acid, the homologue was incorporated both intact and partially degraded prior to incorporation into protein. Alanine was the major L-azetidine 2-carboxylic acid catabolite.

Regulation of herpesvirus macromolecular synthesis: sequential transition of polypeptide synthesis requires functional viral polypeptides

It was previously shown that virus-specific polypeptides made in HEp-2 cells infected with herpes simplex 1 form three groups designated alpha, beta, and gamma whose synthesis is coordinately regulated and sequentially ordered. This report shows that one or more functional alpha polypeptides are necessary to turn on the synthesis of beta and gamma groups, and conversely, one or more polypeptides in the latter groups turn off the synthesis of alpha polypeptides. Specifically, infected cells maintained in medium containing either canavanine, an analogue of arginine, or azetidine-2-carboxylic acid an analogue of proline and hydroxyproline, synthesized alpha polypeptide at rates comparable to maximal rates in untreated infected cells but did not undergo the normal transition to beta and gamma polypeptide synthesis. The transition to gamma polypeptide synthesis and shut-off of synthesis of earlier polypeptide groups proceeded normally if addition of canavanine was delayed until at least 4-5 hr after infection. Addition of canavanine after the onset of beta and gamma polypeptide synthesis, i.e., between 2 and 3.5 hr after infection, resulted in sustained, simultaneous synthesis of all three polypeptide groups, a phenomenon not seen in untreated infected cells. Canavanine-treated infected cells, synthesizing alpha polypeptides, recovered the capacity to make beta and gamma polypeptides after removal of the analogue, but only after a 1-to 2-hr delay compared with infected untreated cells. The data indicate that the on and off controls inherent in the cascade regulation of viral polypeptide synthesis are mediated by one or more polypeptides in each group at transcriptional or post-transcriptional levels.

[The effect of an L-proline analogue, L-azetidine-2-carboxylic acid, on the phenotypic differentiation of the somite mesenchyme of chick embryos]

L-azetidine has been utilized in vivo and in vitro. In vivo, the substance is injected into the yolk sac of young embryos. It inhibits the secretion of the periaxial extracellular material resulting in a lack of several vertebrae. In young vertebral primordia, cultured on L-azetidine, the myotome cells differentiate into myoblasts whereas the sclerotome cells remain undifferentiated. In older primordia, the sclerotome cells give rise to abnormal cartilage but the myotome cells seldom differentiate.