Experiments are described which show that (a) deaminase is inhibited by its own product, the linear hydroxymethylbilane; (b) this inhibition of uptake of porphobilinogen (PBG) is competitive, affecting KM but not Vmax.; (c) addition of synthetic hydroxymethylbilane to deaminase sets up the covalently bound linear tetrapyrrolic system on the enzyme; (d) it is the binding of the next PBG unit after construction of the linear tetrapyrrole which causes release of hydroxymethylbilane from deaminase; (e) the linear tetrapyrrole bound to deaminase is only slowly removed in the presence of a large excess of cosynthetase; (f) when the first PBG unit is taken up by deaminase it is covalently bound to the enzyme with release of one mole of ammonia; (g) the tentative conclusion is reached that this covalent binding occurs via the ε-amino group of lysine; (h) deaminase has no effect on the rate of formation of uro'gen-III from hydroxymethylbilane by cosynthetase.
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Battersby et al. (1983) studied this question.