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

Male albino rats (110-120 g) 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 alpha-ketoacids (9.4%) (BCKA-diet). Pair-fed controls received an isocaloric diet (AA-diet) which contained the three branched-chain amino acids (1.4%) instead of the alpha-ketoacids (nitrogen = 1.2%). A third group was fed on a standard diet. Measuring rates of incorporation of radioactive leucine, valine and their corresponding alpha-ketoacids into liver, kidney, heart and brain proteins of rats fed on a standard diet revealed that in liver, branched-chain alpha-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 alpha-ketoacids results in equal rates of incorporation in kidney and heart proteins. Injecting branched-chain alpha-ketoacids leads to higher incorporation rates in brain than injecting branched-chain amino acids. Thus rates of incorporation of branched-chain alpha-ketoacids differ dependent on the tissue and on the diet applied. They are not consistent with those of branched-chain amino acids.