PURIFICATION AND PROPERTIES OF RAT LIVER HISTIDASE

Rat liver histidase (l-histidine ammonia-lyase, EC 4.3.1.3) has been purified 200-fold. The final preparation was estimated to be about 80% pure and was used to evaluate cofactor requirements and other reaction parameters. Histidase has a molecular weight of 226 000, a Km for histidine of 2.0 · 10-3 M, and a pH optimum of 8.8-9.2. The enzyme is stimulated by glutathione although the latter appears to function as a bivalent anion, not as a sulfhydryl-protecting reagent. Histidase is inhibited by versene, and the inhibition is effectively reversed by both manganese and zinc ions; magnesium is slightly less effective in this regard. Some physiological implications of the histidase reaction are discussed. © 1968.