Retardation of leaf senescence by inhibitors of RNA and protein synthesis

Effects of actinomycin-D (ACT), cycloheximide (CH), rifampicin (RIF) and chloramphenicol (CAP) on senescence of soybean leaf discs were investigated. All inhibitors tested are effective in retarding senescence of soybean leaf discs. However, CH is more effective than ACT, RIF and CAP, suggesting that activation of preexisting, latent metabolic systems present in the cytoplasm piays predominant role in the initiating of leaf senescence. However, the possibility that events taking place in the nucleus or chloroplast are essential for the initiation of leaf senescence cannot be excluded.     

Kinetics for the secretion of nonhelical procollagen by freshly isolated tendon cells

Fibroblasts isolated by enzymic digestion of chick embryo tendons have previously been used to examine the kinetics for the secretion of procollagen (Kao, W. W.-Y., Berg, R. A., and Prockop, D. J. (1977) J. Biol. Chem. 252, 8391-8397). The results indicated that the kinetics approximated the sum of two first order processes with half-times of 14 and 115 min. Here, the same fibroblasts were incubated in the presence of 1.53 mM cis-4-hydroxyproline, an analogue of proline, or in the presence of 0.3 mM alpha,alpha'-dipyridyl, an inhibitor of prolyl hydroxylase, so that the cells synthesized procollagen which could not assume a triple helical conformation characteristic of procollagen. Measurements of the secretion of nonhelical procollagen indicated that the kinetics for secretion differed from the kinetics for the secretion of procollagen and approximated a single first order process with a half-time of approximately 130 min. The nonhelical procollagen synthesized and secreted in the presence of either cis-4-hydroxyproline or alpha,alpha'-dipyridyl consisted of disulfide-bonded pro gamma chains of type I procollagen. The results suggested that the intracellular nonhelical procollagen was present in a single metabolic pool and secretion from this pool occurred with a different rate-limiting step than for helical procollagen. Further results indicated that nonhelical procollagen had a high affinity for prolyl hydroxylase and the affinity for the enzyme was greatly reduced if the procollagen was allowed to assume the triple helical conformation characteristic of normal procollagen. The results are consistent with the hypothesis that the secretion of procollagen is influenced by its conformation-dependent interaction with prolyl hydroxylase or other post-translational enzymes.     

Reconstituted low density lipoprotein: A vehicle for the delivery of hydrophobic fluorescent probes to cells

Previous studies have shown that the cholesteryl ester core of plasma low density lipoprotein (LDL) can be extracted with heptane and replaced with a variety of hydrophobic molecules. In the present report we use this reconstitution technique to incorporate two fluorescent probes, 3-pyrenemethyl-23, 24-dinor-5-cholen-22-oate-3β-yl oleate (PMCA oleate) and dioleyl fluorescein, into heptane-extracted LDL. Both fluorescent lipoprotein preparations were shown to be useful probes for visualizing the receptor-mediated endocytosis of LDL in cultured human fibroblasts. When normal fibroblasts were incubated at 37°C with either of the fluorescent LDL preparations, fluorescent granules accumulated in the perinuclear region of the cell. In contrast, fibroblasts from patients with the homozygous form of familial hypercholesterolemia (FH) that lack functional LDL receptors did not accumulate visible fluorescent granules when incubated with the fluorescent reconstituted LDL. A fluorescence-activated cell sorter was used to quantify the fluorescence intensity of individual cells that had been incubated with LDL reconstituted with dioleyl fluorescein. With this technique a population of normal fibroblasts could be distinguished from a population of FH fibroblasts. The current studies demonstrate the feasibility of using fluorescent reconstituted LDL in conjunction with the cell sorter to isolate mutant cells lacking functional LDL receptors.     

EPR properties of the reaction center of Rhodopseudomas gelatinosa in situ and in a detergent-solubilized form

The photochemical reaction centers from a variety of purple photosynthetic bacteria are composed of a trimer of protein subunits. However, the recently isolated reaction center from Rhodopseudomonas gelatinosa appears to have only two subunits. In this paper we examine the EPR characteristics of the primary photochemical reactants in this species, and compare them with those of other species. Despite of the differences in protein composition, no dramatic differences in EPR properties are seen in vivo, although some interesting effects are seen upon solubilization of the reaction center, which may be related to the unusual lability of the isolated preparation. Perhaps the most noteworthy phenomenon seen in Rps. gelatinosa is the apparent ability of electrons on the reduced intermediary electron carrier to tunnel at low temperatures to the oxidized c-type cytochrome, which has not been seen in other species studied to date.     

A deoxyribonucleic acid-replication intermediate in the growing mosquito.

In previous experiments on growth and aging in the yellow-fever mosquito, Aedes aegypti, a low mol. wt. (500000) DNA species was found in the supernatant fraction after ultracentrifugation of homogenates of rapidly-growing larvae. This DNA species, "sDNA", constituted 30-40% of total DNA in 2-4-day-old larvae, but was less than 5% in older larvae, pupae and adults. We have now isolated and characterized sDNA and initiated experiments to determine its metabolic role. Isolated sDNA has the same physical and chemical characteristics as bulk DNA, "pDNA", and differs only in size. In CsCl isopycnic centrifugation the buoyant densities of sDNA and pDNA were 1.700 and 1.697 g/cm3 respectively. The "melting" temperature of both DNA species was 84 degrees C. Base compositions calculated from these data and other methods were 38.9 mol% of guanine-plus-cytosine for sDNA, and 38.5 for pDNA. Also, the size of newly-synthesized DNA was investigated by pulse-labelling and pulse-chase experiments. In neutral sucrose gradients the labelled DNA component after a 2h pulse had a sedimentation coefficient of about 8S, but after a 4h pulse sedimented in a broad band from 10-19S. In alkaline sucrose gradients a single peak around 7S was observed for pulse times up to 4h. After a 6h pulse and a 1 day "chase", labelled DNA species had sedimentation coefficients ranging from 10-15S in alkaline sucrose, and after a 2-day chase the values were 17-31S, similar to those of pDNA under alkaline conditions. These results suggest that sDNA represents an intermediate form in the replication of DNA in mosquito larvae.     

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.     

Effects of hypophysectomy, growth hormone, and thyroxine on protein turnover in heart.

Cardiac atrophy following hypophysectomy was accompanied by decreased heart content of RNA and polysomes and increased levels of ribosomal subunits, suggesting that protein synthesis was restricted by a reduced supply of ribosomes and an imbalance between rates of peptide-chain initiation and elongation. During perfusion in vitro, provision of palmitate restored the normal balance between rates of initiation and elongation but protein synthesis was lower in hearts of hypophysectomized than normal rats, reflecting the lower RNA content of hearts from hormone-deficient animals. After the period of atrophy had passed, or after treatment with growth hormone and thyroxine, heart RNA content and rates of protein synthesis were equal to or greater than those found in normal hearts. When plasma levels of amino acids, glucose, fatty acids, and insulin, and rates of beating and ventricular pressure development observed in normal and hypophysectomized rats were simulated during in vitro perfusion, hearts from hormone-deficient rats had reduced rates of protein synthesis but unaltered rates of degradation. Cathepsin D activity in heart homogenates (+ Triton X-100) was elevated during cardiac atrophy when expressed per g of tissue but not when expressed per heart.