Joan P. Holbrook

Age-associated changes in activities of rat hepatocytes. I. Protein synthesis.

Experiments involving simultaneous comparisons of cells from two animals establish an age-dependent decline in protein synthesis. Specific radioactivities of fractions from cells of aged animals (20+ months) are depressed 60% to 80% versus those from younger animals (2 to 14 months). Isotope ratio patterns derived by PAGE indicate that formation of a 20000 MW polypeptide of cytoribosomal origin is even more strongly and specifically depressed.

Studies on a cytochrome oxidase antibody. III. Cross reactivity

A rabbit serum antibody induced in response to injections of beef heart cytochrome oxidase has been demonstrated to be capable of the quantitative precipitation of a solubilized rat liver cytochrome oxidase.

Studies of phosphorus metabolism by isolated nuclei. X. Nucleolar nucleosidediphosphatase activity.

Abstract 1. 1. Isolated rat liver nucleoli catalyze formation of P i with both ribo- and 2-deoxyribonucleoside diphosphates as substrates. Nucleolar GDPase activity is maximal at pH 7.2 to 7.8 and 40°; it is stimulated maximally by magnesium or manganese. The principal products of nucleolar GDPase are GMP and P i ; no evidence has been obtained for a contribution to this activity by polynucleotide phosphorylase. 2. 2. Nucleoli also contain glucose-6-phosphatase and inorganic pyrophosphatase. Although such a content of phosphatases is analogous to that of microsomes, the absence of microsomes in the isolated nucleoli, and the non-identity of the nucleolar phosphatases with microsomal phosphatases, has been established by several means. 3. 3. The results of this work verify previous histochemical demonstrations of nucleoside diphosphatase activity in liver and hepatoma nucleoli.

Age-associated changes in rat hepatocytes: An early decline in synthesis of a specific protein by endoplasmic reticulum

Isolated hepatocytes from rats of disparate age were compared for their capacity to incorporate radioisotopic leucine into proteins of subcellular fractions. Determinations of isotope ratios of component microsomal proteins derived from such experiments reveal an age-associated decline in incorporation of leucine into a specific protein of the endoplasmic reticulum. The onset of this decline is detectable at as early as 6 months of age and the depression of synthesis is complete in animals 20 months of age or older.

Age-associated changes in rat hepatocytes: Studies of the character of a polypeptide subject to diminished synthesis at an early age

We have previously reported the observation of an age-dependent aberration in synthesis of a 20000 MW polypeptide (P20000) by isolated hepatocytes. Studies of the post-synthetic distribution of P20000 within the hepatocyte establish that the bulk of P20000 synthesized in vitro remains in association with the endoplasmic reticulum, or can be isolated with the microsomes derived therefrom. No evidence was obtained for an accrual with time of P20000 by the nucleus, mito. chondria or cell sap. An accumulation of P20000 in the secreted protein fraction was observed, but the net secretion of P20000 with time was found to be only a minor fraction of that predictable from the extent of labeling of the microsomal complement of P20000. This result is in sharp contrast to simultaneous measurements of the secretion of albumin. Stability of the radioisotope content of P20000 in the presence of cycloheximide precludes turnover or degradation as a basis for these observations. It is concluded that P20000 is a protein of the liver endoplasmic reticulum. Comparison of hepatocytes from male and female animals establishes that synthesis of P20000 is a property of the male animal.

Effects of Chronic Ethanol Feeding on Rat Hepatocytes

Ethanol was fed at a level of 35% of total caloric intake to mature male rats for intervals of one to 38 days. Mitochondrial content of cytochrome c oxidase per mg protein was maximally depressed, to 40% of control values, after 10 days feeding. After 15 days of ethanol feeding, leucine incorporation into total hepatocyte protein, as well as into total intracellular (subcellular fractions) or extracellular (secreted) proteins was substantially inhibited. At the level of total protein, alterations in isotope ratio as a consequence of ethanol feeding were only manifest in the secreted proteins. However, when component proteins of subcellular fractions were separated by gel electrophoresis, two specific early effects of ethanol feeding on protein synthesis by the endoplasmic reticulum could be detected. Aberrant isotope ratios established that synthesis of two entities, of 20000 and 50000 apparent molecular weight, was depressed after intervals of one and five days, respectively, feeding of 10% ethanol.