In vitro protein degradation measured by differential loss of labeled methionine and 3-methylhistidine: The effect of Insulin

Effects of cimaterol, a beta-adrenergic agonist, on protein metabolism in rats

Fractional accretion rates of total body 3-methylhistidine containing proteins (actin and myosin) were elevated 40% to 120% in rats fed a high-carbohydrate diet containing 10 or 100 ppm cimaterol for 1 week. Fractional degradation and fractional synthesis rates of these proteins were examined by measuring total body 3-methylhistidine content and urinary excretion of 3-methylhistidine. Consumption of the diet containing 100 ppm cimaterol for 1 week caused a 25% reduction in fractional degradation rates and a concomitant 32% increase in fractional synthesis rates of 3-methylhistidine containing proteins. Effects of cimaterol on fractional accretion, degradation, and synthesis rates of 3-methylhistidine containing proteins diminished after 1 week. Cimaterol failed to influence plasma insulin, triiodothyronine, or corticosterone concentrations. The dramatic increase in accretion of 3-methylhistidine containing proteins observed during the first week rats are fed diets containing cimaterol is caused by reciprocal action on protein degradation and synthesis.

Intracellular site of insulin action: mitochondrial Krebs cycle

Effect of insulin on the oxidation of carbons-1,4 and -2,3 of succinate and their incorporation into protein were studied in isolated rat hepatocytes and diaphragm muscle pieces. Oxidation of carbons-2,3 of succinate and their incorporation into hepatocyte protein were stimulated significantly by insulin. Insulin had only a trivial effect on 14CO2 formation from the carboxyl carbons of succinate. These data suggest that insulin affects only those carbons of succinate that are metabolized in the intramitochondrial Krebs cycle.

Dynamic aspects of amino acid metabolism in alloxan-induced diabetes and insulin-treated rabbits: in vivo studies with 15N and gas chromatography-mass spectrometry

The present study was designed to determine the effect of alloxan-induced diabetes in rabbits on L-[15N]alanine and [15N]glycine kinetic parameters. This process was measured by single-dose administration of 15N-labeled amino acids to postabsorptive control rabbits and alloxan-induced diabetics and insulin-treated diabetic rabbits. Gas chromatography-mass spectrometry was used to determine the 15N enrichment of plasma glycine and alanine. Glycine and alanine pools and turnover rate constants were estimated from isotope enrichment time decay curves. The data from the present study indicate that plasma glycine and alanine turnover rate constants increased by 25-50% after alloxan administration but pool sizes showed only little changes, resulting in highly significant increases in fluxes and metabolic clearance rates of both alanine and glycine following alloxan administration; single-dose crystalline insulin or protamine zinc insulin treatment failed to restore the turnover rate constants of glycine or alanine toward control values and caused a depletion of 50% in glycine pool size; 7 days prolonged treatment with protamine zinc insulin restored alanine and glycine fluxes and metabolic clearance rates towards control postabsorptive values; and the reduction in flux values following insulin treatment is consistent with the reduction in the plasma glucose levels in rabbits. The data suggest that the regulatory mechanisms for uptake and metabolism of circulating glycogenic amino acids no longer are operative as a consequence of insulin deficiency following alloxan administration. Exogenous insulin restored the activity of the regulatory mechanism toward the postabsorptive control state.

Anabolic regulation of gluconeogenesis by insulin in isolated rat hepatocytes

The role of substrate availability in the regulation of gluconeogenesis in isolated rat hepatocytes was studied using [U-14C]alanine as a tracer in the presence of different concentrations of L-alanine in the incubation medium. At low alanine concentrations (0.5 mM) insulin decreased the 14C incorporation into the glucose pool and increased the incorporation of tracer carbons into the protein and lipid pools and into CO2. The net radioactivity lost from the glucose pool was only a small percentage of the total increase in the activity of the protein, lipid, CO2, or glycogen pools, supporting the notion that the effect of insulin in diminishing gluconeogenesis is secondary to its effects on pathways using pyruvate. At higher concentrations of alanine (2.5, 5.0, and 10.0 mM) in the incubation medium insulin increased the movement of alanine carbons into protein and glucose. This suggests that at higher substrate concentrations the ability of the liver to synthesize proteins is overwhelmed and the pyruvate carbons are forced into the gluconeogenesis pathway. These results were further confirmed by using [U-14C]lactate. The increases in observed specific activity of glucose following insulin administration would not be possible if insulin acted by affecting the activity of any enzyme directly involved in the formation or utilization of pyruvate, most of which have been proposed as sites of insulin action. Data presented show that insulin "inhibits" gluconeogenesis by affecting a change in substrate availability.

The roles of insulin and glucagon in the regulation of amino acid turnover rate and pool size: in vivo study with [15N]glycine and gas chromatography--mass spectrometry

Gas chromatography--mass spectrometry analysis of plasma amino acid derivatives has been used to determine the 15N enrichment time decay curves of plasma glycine following a single dose administration of [15N]glycine in untreated and insulins-, glucagon-, and cycloheximide-treated rabbits. The present study indicated the following: (a) Increases of 80 and 50% in plasma glycine disappearance rate constants occurred in insulin- and glucagon-treated rabbits as compared with control postabsorptive rabbits; (b) The hormones in the intact rabbits caused a significant depletion in glycine pool size, which led to a moderate reduction in the fluxes of glycine. (c) A significant reduction in glycine turnover rate constants and pool size was noted at 3 and 24 hr following the administration of a sublethal dose of cycloheximide and a restoration towards control postabsorptive values was observed 48 hr after cycloheximide administration. (d) Sublethal doses of cycloheximide inhibited by 60 and 90% the stimulatory action of insulin and glucagon on plasma glycine disappearance, respectively. The present data suggest that both insulin and glucagon may act directly on plasma glycine disappearance rates. The stimulatory action of insulin differs from the action of glucagon in that it is not completely blocked by cycloheximide. Presumably glucagon and insulin modify the glycine transport system at different sites or by a different mechanism.