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L Reshef

Publications and source records attributed to L Reshef.

At least 55 records · Page 3Linked to original sources
PubMed1974

Degradation of phosphoenolpyruvate carboxykinase (guanosine triphosphate) in vivo and in vitro.

1. Phosphoenolpyruvate carboxykinase (GTP) in the cytosol fraction of liver was labelled in young rats by the injection of [(3)H]leucine and then isolated with specific antibody. Antibody-antigen precipitates from ;pulse'-labelled animals and from animals in which the content of radioactive enzyme had been decreased by a period of degradation were separated by electrophoresis on sodium dodecyl sulphate-polyacrylamide gels. No radioactive breakdown products were found. 2. (3)H-labelled phosphoenolpyruvate carboxykinase (GTP) was purified from rat liver and used to measure degradation in vitro. There was first a loss of catalytic activity, then a disappearance of immunological activity and finally a loss of solubility before any evidence of proteolytic cleavage. Proteolytic-cleavage fragments, when found, were also insoluble. 3. An analysis of the subcellular location of enzyme inactivation showed that phosphoenolpyruvate carboxykinase (GTP) was stable when incubated with liver cytosol fraction and was inactivated most rapidly by the microsomal fraction. 4. We propose that denaturation of the enzyme is the rate-limiting step in degradation in vivo, and precedes proteolytic cleavage when the enzyme is incubated with liver preparations in vitro.

Animals↗
PubMed1974

Synthesis of phosphoenolpyruvate carboxykinase (guanosine triphosphate) by isolated liver polyribosomes.

1. Phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) was synthesized by postmitochondrial supernatants of rat liver in the presence of appropriate salts, an energy supply and [(3)H]leucine. Synthesis of enzyme released from polyribosomes was detected by immunoprecipitation with specific antibody followed by electrophoresis of the dissolved antibody-antigen precipitates on sodium dodecyl sulphate-polyacrylamide gels in the presence of a (14)C-labelled enzyme marker. 2. Enzyme synthesis in vitro occurs predominantly on free rather than bound polyribosomes. 3. Starved animals in which de-induction of phosphoenolpyruvate carboxykinase (GTP) had been initiated by re-feeding for 2h had a markedly decreased rate of enzyme synthesis, whether the measurements were made after injection of radioactive leucine into the intact animal or if synthesis was determined in vitro. 4. The low rate of enzyme synthesis by liver polyribosomes from re-fed animals was not due to the absence of soluble factors, nor could it be increased by the addition of cyclic AMP to the protein synthesis system. 5. Phosphoenolpyruvate carboxykinase (GTP) synthesis in vitro is diminished relative to total protein synthesis when the postmitochondrial supernatant is kept at 0 degrees C for several hours before measurement of protein synthesis. Since this effect is blocked by heparin, it is probably caused by selective ribonuclease attack on enzyme mRNA. 6. De-induction of phosphoenolpyruvate carboxykinase (GTP) is tentatively explained as being due to a transcriptional block in specific mRNA synthesis, followed by rapid degradation of existing message.

Animals↗
PubMed1974

Rapid loss of translatable messenger RNA of phosphoenolpyruvate carboxykinase during glucose repression in liver.

The rate of synthesis of phosphoenolpyruvate carboxykinase (EC 4.1.1.32) in rat liver decreased with a half-life of 30 min after fasted rats were refed either a chow diet or glucose. A requirement for both glucose and insulin to bring about this rapid deinduction was shown, as well as the ability of dibutyryl adenosine 3':5'-cyclic monophosphate to block the decrease in enzyme synthesis. Estimates of the stability of messenger RNA of phosphoenolpyruvate carboxykinase were made by using the inhibitors actinomycin D and cordycepin to block further messenger RNA synthesis, and then measuring the decrease in specific enzyme synthesis. It is suggested that the use of actinomycin D yields an overestimation of the template stability. The results with cordycepin imply that the enzyme messenger RNA has a short half-life of approximately 1 hr. Thus, it is possible that deinduction may proceed by way of a decrease in messenger RNA production, leading to a rapid fall in the synthesis of phosphoenolpyruvate carboxykinase.

Animals↗
PubMed1972

The initial synthesis of proteins during development. Phosphoenolpyruvate carboxylase in rat liver at birth.

1. A specific antibody, prepared by immunizing rabbits with phosphoenolpyruvate carboxylase (EC 4.1.1.32) purified from adult rat liver, was used to study the appearance of this enzyme in livers from developing rats. 2. Although some inactive precursor of the enzyme may be present in foetal liver, the amount is not sufficient to account for the enzyme appearance at birth. 3. The rate of phosphoenolpyruvate carboxylase synthesis relative to other cytosol proteins increases 20-fold from the foetus to the 1-day-old rat. The high rate of synthesis was maintained at least until 3 days after birth. 4. There was no measurable degradation of phosphoenolpyruvate carboxylase during the first day after birth. During this period the hepatic enzyme content increased 12-fold. 5. When phosphoenolpyruvate carboxylase attained a constant activity in the liver of rats 2 days after birth the half-time of degradation was approx. 13h. 6. We suggest that the pattern of changes occurring during appearance of phosphoenolpyruvate carboxylase is similar to substrate-induced enzyme induction in bacteria.

Animals↗
PubMed1972

The interaction of catecholamines and adrenal corticosteroids in the induction of phosphopyruvate carboxylase in rat liver and adipose tissue.

Catecholamines induced an increase in the activity of rat adipose tissue and liver phosphopyruvate carboxylases that was maintained for 48h. The response of adipose tissue phosphopyruvate carboxylase was blocked by actinomycin D, corticosteroids and propranolol, whereas corticosteroids and propranolol did not affect the liver enzyme. Cortisol phosphate, like actinomycin D, interfered only with the initiation of the increase in enzyme activity caused by noradrenaline, but not with the process of enzyme accumulation. In contrast, cycloheximide was effective in blocking enzyme induction throughout the course of the catecholamine effect. Adrenocorticotrophic hormone caused a short-term induction of adipose tissue phosphopyruvate carboxylase, which could be blocked by propranolol. Hepatic phosphopyruvate carboxylase, but not the adipose tissue enzyme, was induced by dibutyryladenosine 3':5'-cyclic monophosphate and by glucagon. Both nicotinic acid and nicotinamide decreased the normal induction of adipose tissue phosphopyruvate carboxylase caused by starvation, but only nicotinamide increased the activity of the liver enzyme.

Adipose Tissue↗
PubMed1970

Immunochemical studies with soluble and mitochondrial pyruvate carboxylase activities from rat tissues.

1. Pyruvate carboxylase (EC 6.4.1.1), purified from rat liver mitochondria to a specific activity of 14 units/mg, was used for the preparation of antibodies in rabbits. 2. Tissue distribution studies showed that pyruvate carboxylase was present in all rat tissues that were tested, with considerable activities both in gluconeogenic tissues such as liver and kidney and in tissues with high rates of lipogenesis such as white adipose tissue, brown adipose tissue, adrenal gland and lactating mammary gland. 3. Immunochemical titration experiments with the specific antibodies showed no differences between the inactivation of pyruvate carboxylase from mitochondrial or soluble fractions of liver, kidney, mammary gland, brown adipose tissue or white adipose tissue. 4. The antibodies were relatively less effective in reactions against pyruvate carboxylase from sheep liver than against the enzyme from rat tissues. 5. Pyruvate carboxylase antibodies did not inactivate either propionyl-CoA carboxylase or acetyl-CoA carboxylase from rat liver. 6. It is concluded that pyruvate carboxylase in lipogenic tissues is similar antigenically to the enzyme in gluconeogenic tissues and that the soluble activities of pyruvate carboxylase detected in many rat tissues do not represent discrete enzymes but are the result of mitochondrial damage during tissue homogenization.

Adipose Tissue↗
PubMed1967

Effect of propionate on lipogenesis in adipose tissue.

The metabolism of propionate in adipose tissue and its effect on lipogenesis was investigated. Fasting induced changes in propionate metabolism of adipose tissue, drastically reducing higher fatty acid synthesis and increasing glyceride-glyerol formation from low concentrations of propionate (0.25 mM). Propionate also promoted lipogenesis from acetate-1-(14)C in tissues of fasted rats, while it inhibited lipogenesis and CO(2) formation from acetate in the fed animal. Treatment with actinomycin D or ethionine abolished both the increased glyceride-glycerol formation from propionate and the promoting effect on lipogenesis from acetate. Synthesis of long-chain fatty acids from propionate-1-(14)C was increased by actinomycin treatment. The change in propionate metabolism induced by fasting is, however, not entirely due to its conversion to glyceride-glycerol, since the latter was almost completely blocked by malonate while part of the promoting effect on fatty acid synthesis persisted.

Acetates↗