Practical methods to estimate whole body leucine oxidation in maple syrup urine disease

We report the comparison of noninvasive methods to estimate whole body leucine oxidation in patients who have maple syrup urine disease. We used both an i.v. and an oral bolus of L-[1-13C]leucine and quantitated 13CO2 in expired air. Both methods differentiated patients with maple syrup urine disease from heterozygous and control subjects. Eight patients, whose disease differed in clinical severity, were selected for study and had a range of impaired values for whole body leucine oxidation. Six h after an i.v. bolus dose of L-[1-13C]-L-leucine, 13CO2 recoveries ranged from 0.8 to 19.7%. Three of the eight patients had significant increases in 13CO2 production after supraphysiologic thiamine therapy. After the oral dose of L-[1-13C]leucine, homozygous affected children produced less 13CO2 than normal, age-matched, childhood controls. In addition, the oxidation of orally administered L-[1-13C]leucine was reduced significantly in adult heterozygotes compared with adult controls. The proportion of whole body leucine oxidation by affected children was comparatively greater than that by their cultured cells, but cellular oxidation correlated significantly with whole body oxidation of leucine among affected patients. We conclude that these simplified analyses of whole body leucine oxidation define the degree of impaired branched-chain alpha-ketoacid dehydrogenase activity in patients with differing types of maple syrup urine disease and distinguish the subpopulation who might benefit from thiamine supplementation.