A Thesis submitted to the Faculty of Medicine, University of the Witwatersrand, Johannesburg, for the Degree of Master of Science JOHANNESBURG 1986 / The adenosine deaminase enzyme from a Xhosa tribesman has been characterized. Red blood cell activity levels were 6-9% of normal whereas his white cell ADA levels were about 30% of normal. The enzyme's stability at 57°C was shown to be greatly reduced suggesting a mutation resulting in an enzyme with reduced stability in vivo. It was concluded that the discrepancy in ADA activity levels between red and white blood cells was due to the red cells being anucleate. The proband's residual ADA was found to have a Michaelis Constant (K ) for adenosine m of 47.9 ♦ IS.BuM, a value which is not significantly different from that of normal ADA (51.7 ± 11.4ufl). Red cell deoxy-ATP levels were measured and found to be elevated two-to-three times over normal levels. Red cells from ADA-deficient patients with severe combined immunodeficiency (SCID) have been reported with deoxy-ATP levels elevated about 1 000 times. It was concluded that the slight elevation of deoxy-ATP levels in the proband were too low to have any noticeable effect on functions of his immune system. Starch gel electrophoresis of red cell ADA from members of the proband's family in conjunction with red cell ADA activity levels suggested that both parents carried a gei e for 'partial' ADA deficiency, both of which had been inherited by the proband as well as one of his sibs. Isoelectric focusing studies suggested that the two, parental AUA partial deficiency genes were different from one another. It was also found that another rare allele of ADA, possibly ADA ',was segregating within the same family although this event appaars to be unconnected with the ADA partial deficiency.