Comparative studies between rates of incorporation of branched-chain amino acids and their .ALPHA.-ketoanalogues into rat tissue proteins under different dietary conditions.

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Male albino rats (110-120g) were fed for 10 days on an amino acid diet low in nitrogen (nitrogen =1.05%) devoid of valine, leucine and isoleucine and supplemented with branched-chain a-ketoacids (9.4%) (BCKA-diet). Pair-fed controls received an isocaloric diet (AAdiet) which contained the three branched-chain amino acids (1.4%) instead of the a-ketoacids (nitrogen=1.2%). A third group was fed on a standard diet. Measuring rates of incorporation of radioactive leucine, valine and their corresponding a-ketoacids into liver, kidney, heart and brain proteins of rats fed on a standard diet revealed that in liver, branched-chain c ketoacids are incorporated to a lesser extent than the corresponding amino acids. The same was also observed with the BCKA-diet, while the AA-diet reduced BCAA incorporation with the consecutive improvement of the relative incorporation of BCKA over that of BCAA. Injection of branched-chain amino and α-ketoacids results in equal rates of incorporation in kidney and heart proteins. Injecting branched-chain α-ketoacids leads to higher incorporation rates in brain than injecting branched-chain amino acids. Thus rates of incorporation of branchedchain α-ketoacids differ dependent on the tissue and on the diet applied. They are not consistent with those of branched-chain amino acids.
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