Identification and tissue-specific expression of a NADH-dependent activity of dihydropyrimidine dehydrogenase in man

Homogenates of human liver and human fibroblasts were able to convert thymine into dihydrothymine in the presence of NADH whereas almost no NADH-dependent activity could be detected in human lymphocytes. The different tissue distribution of the NADH-dependent activity suggests that different types of human dihydropyrimidine dehydrogenase exist. Both types of human liver dihydropyrimidine dehydrogenase showed a comparable affinity towards thymine, NADH and NADPH. Only a ten-fold lower Vmax value was observed for the NADH-dependent enzyme. During partial purification of the NADPH-dependent enzyme, on a 2', 5' - ADP Sepharose 4B column, the NADH-dependent activity was completely lost. Neither type of activity was retained on a 5' - AMP Sepharose 4B column.