INHIBITORY EFFECT OF PROPIONIC-ACID ON THE 5-AMINOLEVULINIC ACID (ALA) DEHYDRATASE FROM RHODOBACTER-SPHAEROIDES AND ALA EXCRETION

The role of propionic acid as an inhibitor of 5-aminolevulinic acid dehydratase (ALAD) from Rhodobacter sphaeroides was studied to elucidate the high extracellular accumulation of 5-aminolevulinic acid (ALA) in a volatile fatty acid (VFA) medium. Like levulinic acid (LA), propionic acid inhibited ALAD as a competitive inhibitor, but it's K-i value (1.85 mM) was larger than that of LA (0.48 mM). The inhibitory effects of acetic and butyric acids in the VFA medium were negligible. Using the kinetic model equation of competitive inhibition, the relative enzyme activities were estimated for two cases-the presence of only LA and the presence of LA plus propionic acid. When propionic acid was present with LA, ALAD activity was reduced 60-50% as compared with the case of LA only. The high accumulation of ALA in a VFA medium might therefore be caused by the additional inhibitory effect of propionic acid for ALAD together with the inhibition of LA.