Altered forms of mammalian nucleoside diphosphate reductase from mutant cell lines

Abstract

The synthesis of DNA requires a continuous and balanced supply of the four deoxyribonucleoside triphosphates that originate from the direct reduction of the 2'-carbon atom on the ribose moiety of ribonucleotides. In mammalian cells this reduction occurs at the diphosphate level in the presence of ribonucleoside diphosphate reductase (EC 1.17.4.1), and with one exception, a single enzyme activity (nucleoside diphosphate kinase) is required to provide the four substrates for DNA polymerase. The exception is dTTP which arises from reduction of either UDP or CDP, with the introduction of a methyl group at the monophosphate level. It should be noted that ribonucleoside diphosphate reductase is solely responsible for conversion of ribonucleotides to deoxyribonucleotides, and therefore occupies a crucial position in the synthesis of DNA. The reaction is shown in Fig. 1. Two small proteins, thioredoxin through the thioredoxin reductase system and glutaredoxin via glutathione and glutathione reductase function as hydrogen carriers in the reaction. For more information on the thioredoxin and glutaredoxin systems the reader is referred to previous reviews (Thelander and Reichard, 1979; Holmgren, 1981).