BIOSYNTHESIS OF PORPHYRINS IN WHEAT LEAVES: II. 5-AMINOLAEVULINATE HYDRO-LYASE

Abstract

5-Aminolaevulinate hydro-lyase (EC 4.2.1.24), which catalyzes the formation of porphobilinogen from 5-aminolaevulinate (5-ALA), was isolated from wheat leaves and partially purified. The enzyme was specific for 5-ALA, sulfhydryl-dependent, and required divalent cations for maximum activation. Pyrophosphate, EDTA, and ATP were strongly inhibitory. Laevulinate, but not ethyl laevulinate, was also an inhibitor. The pH optima were 7.5–7.6 in Tris and 7.2–7.8 in phosphate buffer. Michaelis constants for 5-ALA, Mg++, and Mn++were 1 × 10−3 M, 3.1 × 10−4 M, and 3.7 × 10−5 M respectively. The enzyme was localized partially in the chloroplasts and also in 'remaining particles'. Inhibition studies indicated that a carbonyl group γ to an ionized carboxyl group is necessary for binding the substrate to the active site.