MECHANISMS OF THIAMINE STIMULATED BRANCHED CHAIN AMINOACID METABOLISM

Thiamine pyrophosphate (TPP) is a coenzyme of branched chain ketoacid dehydrogenase (BCKA'ase). Responses to its precursor Vitamin B1 occurred in a 7 year-old male with maple syrup urine disease (MSUD). B1 (100 mg/day for 3 weeks) decreased urinary BCKA excretion from 0.9 to 0.2 gm/gm creatinine and increased decarboxylation of branched chain aminoacids (BCAA) by his lymphocytes (WBC) from 2-5% to 17-26% of control. During the subsequent 2 years on B1 therapy, he had no ketoacidemia, and progressed developmentally. When urinary BCKA were below 0.2 gm/gm creatinine, further stimulatory effects of B1 were not seen. BCKA'ase activity in mitochondria isolated from his cultured skin fibroblasts was augmented by 0.2 mM TPP and Mg2+ only at αketo-β-methylvalerate (KMV) concentrations above 5.0 mM. TPP had no stimulatory effect on control mitochondria. However, B1 reduced maximal plasma isoleucine (lleu) (20±4 to 9±2 mg%) reached by 3 controls after 150 mg/Kg lleu, and increased their WBC BCAA decarboxylation 45 to 260%. TPP and Mg2+ prevented the decline (7 to 0.2 nanomoles CO2/mg/15 min.) of KMV BCKA'ase activity in inner membrane-matrix from control mitochondria after 100 minutes of preincubatton. We propose that TPP enables an alternate low affinity pathway for KMV utilization when BCKA'ase is impaired, as in MSUD, and augments KMV BCKA'ase by decreasing the rate of normal enzyme degradation.