Protein turnover is a cyclic process with a net loss of
protein in the (catabolic) fasted state and a net gain in the (anabolic) fed state. In
maple syrup urine disease (MSUD) the early block of degradation of the
branched-chain amino acids (BCAA) brings about the opportunity for evaluation of the diurnal variation in net
protein anabolism and catabolism by studying cyclic changes in the plasma concentrations of BCAA. The alterations in plasma BCAA in a 3-y-old boy with classical MSUD were studied in the fed and fasted state over a period of 19 months. For each
amino acid a total of 34 data pairs was calculated. The plasma concentrations of the BCAA
leucine,
valine and
isoleucine were constantly higher in the fasted than in the fed state. Plasma concentrations of
alloisoleucine, being a non-
protein amino acid, did not participate in cyclic changes. In contrast, the
essential amino acid pair
tyrosine and
phenylalanine increased after meals. The fasting concentration of
alanine increased after feeding, while
glycine did not change significantly. Healthy subjects show a decrease in all
amino acids in the fasted (mild catabolic) state and an increase in the fed state. These findings in MSUD suggest a net decrease in non-BCAA as result of a greater rate of
amino acid oxidation rate than of
protein breakdown and a net entry of BCAA into plasma in the fasted state due to the specific metabolic block. Such changes in
amino acid plasma pools have to be taken into account during monitoring of treatment and especially when in vivo
leucine oxidation is assessed.