Ethanol inhibition of rabbit reticulocyte haem synthesis at the level of delta-aminolaevulinic acid synthetase.

Ethanol inhibition of rabbit reticulocyte synthesis occurs as a result of a decrease in haem synthesis. The present study therefore was undertaken in order to localize the inhibitory site of ethanol on the haem biosynthetic pathway. Ethanol (0.05--0.15 M) inhibition of reticulocyte protein synthesis was prevented by simultaneous incubation with 0.025--1 mM delta-aminolaevulinic acid (ALA). Ethanol inhibited both 14C-glycine and 14C-ALA incorporation into haem. However, the extent of haem formation with 14C-ALA as substrate in the presence of ethanol was still equal to that when 14C-glycine was used. These data suggest that ethanol inhibits the haem synthetic pathway at several loci, but that the decrease in haem synthesis, responsible for the decrease in protein synthesis, is due to the inhibition at the rate-limiting enzyme, delta-aminolaevulinic acid synthetase (ALA-S). To confirm this, ALA-S activity was then directly measured in intact reticulocytes, and it was shown that ethanol indeed inhibited its activity. The inhibition of ALA-S was prevented by 10(-4) M dibutyryl cyclic AMP (db cAMP) or theophylline, agents which elevate intracellular cAMP and which have previously been shown to prevent and reverse ethanol inhibition of haem and protein synthesis. Thus, it appears that cAMP protects against ethanol toxicity by preventing inhibition of ALA-S.