The regulation of protein metabolism in the human heart has not previously been studied. In 10 postabsorptive patients with coronary artery disease, heart protein synthesis and degradation were estimated simultaneously from the extraction ofintravenously infused L-[ring-2,6-3Hjphenylalanine (PHE) and the dilution of its specific activity across the heart at isotopic steady state. We subsequently examined the effect of branched chain amino acid (BCAA) infusion on heart protein turnover and on the myocardial balance of amino acids and branched chain ketoacids (BCKA) in these patients.In nmol PHE/min). BCAA infusion stimulated the myocardial uptake of both BCAA (P < 0.005) and their ketoacid conjugates (P < 0.001) in proportion to their circulating concentrations. Net uptake of the BCAA greatly exceeded that of other essential amino acids suggesting a role for BCAA and BCKA as metabolic fuels. Plasma insulin levels, cardiac double product, coronary blood flow, and myocardial oxygen consumption were unchanged. These results demonstrate that the myocardium of postabsorptive humans is in negative protein balance and indicate a primary anabolic effect of BCAA on the human heart. (J.
In the first paper of this series (Miller & Carpenter, 1964) we reported that the total sulphur amino acid content of meat, fish and whale-meat meals was not directly related to their net protein utilization (NPU) for the rat even when sulphur amino acids had been shown to be limiting. The results were interpreted to indicate a degree of unavailability of the sulphur amino acids, especially in those meals of low NPU. Estimates of 'digestible sulphur amino acids' were found to correlate closely with Recently Ford (1962) has described microbiological procedures for the determination of various individual amino acids, including methionine, in protein concentrates, available to Streptococcus xymogenes. For a series of whale-meat meals his results for available amino acids correlated closely with those for NPU. Such correlations indicate that the protein materials are ranked in the same order by both microbiological and rat procedures, but do not provide evidence that values for amino acids available to Strep. xymogenes are the same as for those available to higher animals.The purpose of the work now described was to determine the availability of methionine in animal protein concentrates by growth assays on the chick and to evaluate the microbiological procedure as a means of predicting the available methionine content for the chick.Guttridge, Lewis & Morgan (1961) and Guttridge & Lewis ( 1 9 6 4~) have described a procedure with chicks for the assay of available methionine, and in our preliminary experiment we used a basal diet modelled on theirs. In subsequent work a basal diet more deficient in methionine was obtained by replacing soya-bean meal with groundnut meal as the main source of amino acids.NPU.
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