BIOTRANSFORMATION OF NITROSO AROMATIC-COMPOUNDS AND 2-OXO ACIDS TO N-HYDROXY-N-ARYLACYLAMIDES BY THIAMINE-DEPENDENT ENZYMES IN RAT-LIVER

The formation of N-hydroxy-N-arylacylamides from nitroso aromatic comp ounds and a-ore acids was investigated using rat liver subcellular fra ctions. Activities were found in both mitochondria and cytosol, except for activities for phenylpyruvate and glyoxylate; the former did not produce N-hydroxy-N-phenylphenylacetamide and the latter nonenzymatica lly produced N-hydroxy-N-phenylformamide with nitrosobenzene (NOB), Th e cytosolic activity of N-hydroxy-N-phenylglycolamide formation was in dicated to be due to transketolase, which utilized hydroxypyruvate as a glycolic aldehyde donor to NOB. With mitochondria, 2-oxo acids (incl uding hydroxypyruvate) served as substrates for the biotransformation of NOB to the corresponding N-hydroxy-N-phenylacylamides. The substrat e preference was 2-oxobutyrate > pyruvate > 2-oxoisovalerate > 2-oxois ocaproate > 2-oxovalerate > 2-oxo-3-methylvalerate, judging from V-max /half-saturating concentration for mitochondria values. The half-satur ating concentrations for NOB were nearly constant. The mitochondrial a ctivity was due to pyruvate dehydrogenase complex and branched-chain 2 -oxo acid dehydrogenase complex (BCDHC). By using partially purified B CDHC, pyruvate and 2-oxobutyrate were found to he common substrates fo r both of the enzymes, and 2-oxoisovalerate was shown to be the most e ffective substrate for BCDHC. Analysis by the Taft equation indicated that the polar effects, rather than the steric effects, of the alkyl g roups of 2-oxo acids are important for BCDHC-catalyzed formation of N- hydroxy-N-phenylacylamides. A positive Hammett constant obtained for t he formation of N-hydroxy-N-arylisobutyramides indicates that an elect ron-withdrawing substituent makes the nitroso compounds susceptible to BCDHC-catalyzed biotransformation.