[Properties of alpha-N-acetylgalactosaminyltransferases in sera of group A and weak A subjects].

Activities of A gene-specified alpha-N-acetylgalactosaminyltransferase in sera of subjects with "normal A" (A1, A2, A1B, A2B) and "weak A" phenotypes (A3, Ax, Aend, Am, Ay and Ael) have been investigated using both 2'fucosyllactose and O red blood cells as exogeneous acceptors with H serological specificity. Among the normal A samples, the enzymatic studies provided two main conclusions: 1) A1 and A2 gene-products respectively found in A1 or A1B and A2 or A2B sera, could be distinguished from each other according to their kinetic properties, namely: (i) optimum pH activity (6.0 or 7.0 respectively); (ii) metal requirement (effect of Mg++); (iii) in vitro conversion of O into A RBC. As a matter of fact, A1 and A1B sera gave high RBC converted titer, but A2 sera lead to very weak or negative values. However an unexpected difference of behavior noticed between A2 and A2B sera,--the latter being a good source of A enzyme for RBC conversion,--suggested a possible interaction between A2 and O gene products on the one hand, A2 and B gene products on the other. 2) A simple and reproducible assay, established on pH ratio dependent values, allows the direct recognition of A1A2 genotypes among A1 subjects, which means that both A1 and A2 enzymes are present in heterozygote sera. Further studies on "weak A" samples sera can be summarized as follows: a) Two kinds of Am bloods--tentatively named AmA1 and AmA2--were identified according to the kinetic properties of their enzymes which were respectively similar to those described in A1 and A2 sera. However, the activity of "Am" enzymes was only 1/2 to 1/3 of that of controls A1 or A2. The genetical background of Am phenotypes was then discussed to the light of results leading to the experimental identification of A2AmA1 genotypes. b) In prolonged incubation times, a very weak A enzyme activity (1/50 to 1/200 of controls) was noticed in Ay sera. Therefore the recessive "yA" gene appears to be not completely silent, but blocks the A enzyme synthesis in most of the cell lines of the organism. c) A3 samples are highly heterogeneous. Two groups of individuals were first identified in which the A enzyme activities was weak (group I, activity 1/8 to 1/20 of controls) or absent (group II). Moreover the occurrence of an A3serum exhibiting strong A enzyme activity (1/2 to 1/3 of controls) and kinetic properties similar to those described in A1 sera suggest the possible existence of a third group. d) In standard or prolonged incubation times, alpha-N-acetylgalactosaminyltransferase seems to be absent in several samples of Ax, Aend or Ael sera. In these cases, as well as in the A3 group II samples sera, no hydrolase or inhibiting substances for transferase activity was demonstrated. Finally, from all these results, the significance of the transferases activities and properties measurements performed directly on whole sera was discussed.