Abstract
Summary 1. ATP:taurocyamine phosphotransferase (EC 2.7.3.4) anci ATP:lombricine phosphotransferase (EC 2.7.3.5), prepared according to the methods described, are homogeneous on ultra-centrifugation. S 20, w values and molecular weights are determined. 2. Determination with p -chloromercuribenzoate or N -ethylmaleimide at pH 7 in the presence of urea, or with 5,5′-dithiobis (2-nitrobenzoic acid) at pH 6.8, revealed 6 SH groups in lombricine kinase and 8–9 in taurocyamine kinase. The latter enzyme exhibits 12 SH groups when titration with p -chloromercuribenzoate is carried out at pH 4.6. 3. The decrease of both enzymic activities is proportional to the amount of N -ethylmaleimide or p -chloromercuribenzoate bound to the enzymes. The stoichiometric titration of SH groups compared with the parallel loss of transferase activity shows that taurocyamine kinase possesses 4 reactive SH groups and lombricine kinase only one. It is concluded that the active site of ATP :guanidino phosphotransferases seems to involve only one SH group.
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