Capacity of human subjects to utilize keto analogues of valine and phenylalanine.

Three adult human subjects were maintained for 7 days (period I) on a protein-free formula diet containing the minimum daily requirements of the eight essential amino acids plus 40 g glycine. During the last 5 days of this period, the average daily nitrogen balances for the three subjects were +0.52, +0.71, and +0.30 g, respectively. During the next 7 days (period II), valine was withdrawn from the diet, and the glycine ration increased by an equimolar amount. During the last 5 days of period II, average daily nitrogen balances declined to -1.82, -1.61, and -1.87 g, respectively. In the final period of 7 days (period III), the keto analogue of valine, alpha-ketoisovaleric acid, was added to the diet in a quantity equimolar to the minimum daily requirement of valine. During the last 5 days of this period, average daily nitrogen balances improved to -0.02, -0.18, and -0.83 g, respectively. Analogous experiments in three subjects involved the withdrawal from the diet of phenylalanine (period II) and replacement by its keto analogue, phenylpyruvic acid (period III). The average daily nitrogen balances were as follows: period I: +1.04, +0.96, and +0.53 g; period II: -1.45, -1.83, and -1.94 g; period III: +0.07, +0.11, and -0.52 g. The data demonstrate that man can convert alpha-ketoisovaleric acid and phenylpyruvic acid to the corresponding essential amino acids, valine and phenylalanine. The efficiency of these conversions is considerably less than 100%.