Porphobilinogen deaminase is unstable in the absence of its substrate

Porphobilinogen deaminase is induced during the dimethyl sulfoxide-mediated differentiation of Friend erythroleukemia cells. We have previously shown that when succinylacetone, a potent inhibitor of porphobilinogen formation, is present during the differentiation process, the induction of the enzyme is apparently suppressed. Here, we provide evidence that, in this condition, porphobilinogen deaminase is synthesized normally but does not accumulate as a consequence of an accelerated turnover. The normal half-life of the protein is 24 h but decreases to 10 h when the formation of its substrate is impaired by succinylacetone. We propose that when the enzyme is covalently bound to its substrate, a normal step in this enzymatic reaction, it is protected from proteolytic degradation, and we show that this new finding is relevant to the human disorder acute intermittent porphyria.