Branched-chain alpha-keto acids isolated as oxime derivatives: Relationship to the corresponding hydroxy acids and amino acids in Maple Syrup Urine Disease

Abstract

Branched-chain α-keto acids can be isolated from serum and urine with good recoveries when treated first with hydroxylamine. Silylation of the keto acid oximes then yields clearly identified peaks after gas chromatography. Serum branched-chain keto acids are increased ten fold above normal in patients with Maple Syrup Urine Disease (MSUD) due to branched-chain α-keto acid dehydrogenase deficiency, when the level of branched-chain amino acids are between the normal range (0.05-0.2 m M) and 1 m M. Keto acids rise another 10–20-fold (i.e., up to 200 times normal), when serum amino acid levels are permitted to rise only twofold above 0.8–1.0 m M. The acute clinical syndrome of MSUD coincides with the latter severe accumulation of keto acids. Removal of branched-chain keto acids by transamination in vivo to the corresponding amino acids seems to be most effective when serum amino acid concentrations are below 1 m M. Keto acids can also be excreted into urine, a mechanism in which they have preference over the corresponding amino and hydroxy acids. Accumulation of branched-chain α-hydroxy acids in MSUD is significant only for α-hydroxy- isovaleric acid, as reported previously by others. Thiamine-responsive MSUD is characterized by greater accumulation of α-keto-B-methyl-valerate relative to the two other branched-chain keto acids when compared with keto acid ratios in classical MSUD. It has been shown by Elsas et al. 21 that thiamine preferentially stimulates the oxidation of isoleucine and its keto acid in cultured skin fibroblasts. Branched-chain keto acid concentrations are sufficient in uncontrolled MSUD to permit inhibition of pyruvate dehydrogenase according to current evidence in vitro. However, there is no abnormal retention of pyruvate or lactate in vivo in the acute clinical syndrome. Dietary control which permits serum branched-chain amino acids to rise above the normal range and yet remain below 0.8–1.0 m M may be as effective as more stringent dietary measures.