Inherited variants of human nucleoside phosphorylase

SUMMARY 1. A method, for the starch gel electrophoresis of human nucleoside phosphorylase (NP) is described. Multiple NP isozymes were found in most human tissues and the best resolution of these isozymes was achieved by electrophoresis in a buffer system containing lithium ions. 2. Tissue to tissue variation in the complexity of the NP isozyme patterns and the examination of NP isozymes in relatively young red cells, suggest that a primary isozymic form of human NP is modified in vivo with generation of several secondary isozymes. The slowest isozyme is probably the primary form in all tissues, the secondary isozymes have greater anodal electro‐phoretic mobilities. Storage experiments carried out with fibroblasts indicate that in vitro modification of the NP isozyme patterns may also occur. 3. Three electrophoretically different genetically determined variants of NP were found in a survey of red cell lysates from 2178 unrelated individuals; one of them was found twice. Family studies showed that these variants occur in individuals heterozygous for a common allele (NP 1 ) and a variant allele (either NP 2 , NP 3 or NP*) at an autosomal locus. 4. Examination of the NP isozymes in cultured fibroblasts and hair follicle cell extracts obtained from individuals with variant red cell phenotypes suggests that NP has a trimeric structure. In vitro hybridization experiments using a freeze‐thaw‐NaCl technique with mouse and human liver and with calf spleen and human liver NP support this view. 5. The molecular size of human NP by gel filtration chromatography is c. 84,000.