SUMMARYThe product of the A blood group gene in the erythrocyte membrane and serum from ‘weak A’ variants was investigated using low and high molecular weight acceptors and compared with common A1and A2blood samples. A reduced enzyme activity was detected, but only in some of the A variants, namely the A3(eight out of eleven), Amor Aysamples. Other sera from A3(three out of eleven), Ax, Aendor Aelindividuals were apparently devoid of enzyme activity.A kinetic study by temperature inactivation of A blood group sera also showed that A enzymes are more labile than B enzymes from BIor BIIsera. Moreover, in one case (A3sample Del.), a very fast inactivation of the A enzyme was observed, suggesting the occurrence of a variant enzyme qualitatively different from the others so far studied.The erythrocyte membrane preparations from all A variants contained no detectable A enzyme activity except for the A1and A2samples, the former being three to five times more active than the latter. The B enzyme activities from four samples of B RBC tested were comparatively stronger than the A enzyme activity of A1RBC.The results were discussed and it was suggested that the synthesis and/or the secretion of the A enzyme in the organism is not uniform from one tissue to another, but could depend on which of the ‘weak A’ alleles or modifier genes is i
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