INHIBITION OF PORPHOBILINOGENASE BY PORPHYRINS IN SACCHAROMYCES-CEREVISIAE

The biosynthesis of uroporphyrinogen III, the precursor of hemes, chlo rophylls, corrins and related structures, is catalyzed by the porphobi linogenase system (PBGase), a complex of two enzymes, PBG-Deaminase (P BG-D) and Isomerase. Although the separate enzymes have been studied i n some detail less work has been performed on the properties of the co mplex. In this study the kinetic behaviour of the enzyme PBGase in a n ormal yeast strain, D273-10B, and its derivative B231 has been investi gated. Uroporphyrinogen formation was linear with time up to 2 hr at 3 7 degrees C. The enzyme complex shows classical Michaelis-Menten kinet ics. From the double reciprocal plots kinetic parameters were estimate d for PBGase and PBG-D. Porphyrins were found to be competitive inhibi tors with respect to porphobilinogen (PBG) and these compounds appeare d to act as inhibitors by forming dead-end complexes with the free enz yme. 5-Aminolevulinic acid (ALA) also inhibited PBGase and this inhibi tion was overcome by addition of levulinic acid (2 mu M). These result s indicate that ALA, is not an inhibitor but acts through its conversi on into porphyrins which are the true inhibitors.