Nucleotidase activities in soluble and membrane fractions of three different mammalian cell lines

Soluble cytoplasmic and membrane fractions were prepared from three cultured mammalian cell lines: 3T3 mouse fibroblasts, V79 hamster lung cells, and human "Cherry" B-lymphoblastoid cells. By using relatively specific nucleotidase assays, together with a phosphotransferase assay, the activities of three different enzymes (low-Km nucleotidase, high-Km nucleotidase, and 5'(3')-nucleotidase) capable of dephosphorylating deoxyribonucleoside 5'-monophosphates were determined in these fractions. The three nucleotidases exist simultaneously in all cell lines, but their relative amounts showed large variations. The 5'(3')-nucleotidase dominated Cherry and 3T3 cells, while in V79 cells equal amounts of this enzyme and the high-Km nucleotidase were recovered. In the membrane fractions, the low-Km nucleotidase was the predominant enzyme. We found no evidence for cell-cycle control of any nucleotidase. We postulated earlier that substrate cycles, involving 5'-nucleotidases and deoxyribonucleoside kinases, provide a mechanism for the regulation of deoxyribonucleotide pools. We suggest that both the low-Km nucleotidase and the 5'(3)-nucleotidase are candidate enzymes for such cycles.