Partial adenosine deaminase deficiency: another family from southern Africa

Adenosine deaminase (ADA) from a partially ADA-deficient Xhosa man has been characterized. This is only the second such case described in southern Africa, the previous one being a Kalahari San ("Bushman"). Red blood cell ADA levels were found to be only 6-9% of normal whereas his white cell ADA levels were much higher at 30% of normal. The stability of the enzyme at 57 degrees C was shown to be greatly decreased indicating a mutation resulting in an enzyme with decreased stability in vivo. The Michaelis constant (Km) for adenosine was found to be normal. Deoxy-ATP levels in the red cells were elevated 2- to 3-times above normal, although this appears to be of no immunological consequence. Starch gel electrophoresis of red cell ADA from family members of the index case, in conjunction with red cell ADA activity levels, suggested that both parents carried a gene for partial ADA deficiency. Isoelectric focusing studies suggested that the two parental partial ADA-deficiency genes were not the same. Electrophoretic studies also revealed that another rare allele of ADA, possibly ADA*5, was segregating within the same family although this event appears to be unconnected with the ADA partial deficiency. A Xhosa population sample was assayed for red cell ADA activity. The results suggested a frequency of 0.015 +/- 0.010 for ADA partial-deficiency alleles, although the number of different alleles involved is not known.