Steady-state kinetics of cabbage histidinol dehydrogenase.

Cabbage histidinol dehydrogenase (HDH) oxidizes L-histidinol to L-histidine through two sequential NAD(+)-linked reactions via an alkaline-labile, L-histidinaldehyde intermediate. The kinetic mechanism of the overall reaction as well as the partial reactions involved in the overall catalysis were investigated at pH 7.2 using L-histidinaldehyde as a substrate. Product inhibition patterns conformed to a Bi Uni Uni Bi Ping Pong mechanism as reported for the HDH from Salmonella typhimurium. Thus, the reaction scheme is ordered with the binding of histidinol first and NAD+ second, and histidine is the last product to be released. The intermediate, L-histidinaldehyde, could be a substrate for both the oxidation and the reduction reactions to produce histidine and histidinol, respectively. L-Histidine was not enzymatically reduced in the presence of NADH, indicating that the reaction to oxidize histidinaldehyde is apparently irreversible. L-Histidinaldehyde exhibited a three times greater binding rate constant than histidinol with a considerably small dissociation constant. These results were in agreement with the observation that histidinaldehyde was not released during the overall reaction. The rate of the reduction of histidinaldehyde to histidinol was almost same as that of the overall oxidation reaction. The overall oxidation from histidinol to histidine proceeded about three times slower than the partial oxidation from histidinaldehyde to histidine, suggesting that the first-half forward reaction is the rate-determining step in the total reaction of cabbage HDH.