NADPH-dependent GMP reductase isoenzyme of human (GMPR2): Expression, purification, and kinetic properties

GMP reductase (EC 1.6.6.8) is the only known metabolic step by which guanine nucleotides can be converted to the pivotal precursor of both adenine and guanine nucleotides. Human GMP reductase has been previously partially purified from erythrocytes and a chromosome 6-linked cDNA has been identified to correspond for encoding human GMP reductase. Here, we reported a distinct cDNA for human GMP reductase isoenzyme isolated from a human fetal brain library, and the GenBank accession number is AF419346. The deduced protein shows 90% identity with human GMP reductase reported (named GMPR1 compared with GMPR2 of this paper) and 69% with E. coli GMP reductase. Comparison of GMPR2 cDNA sequence with human genome indicates the corresponding gene spans about 6.6kb on chromosome 14, which encodes 348 amino acid residues. Northern hybridization analysis indicates a differential and disproportionate expression of mRNAs for GMPR1 and GMPR2, suggesting the existence of distinct molecular species of GMP reductase in human. The apparent Km of GMPR2 for NADPH and GMP are 26.6 and 17.4μM, respectively. This is the first report suggesting the existence of two distinct types of human GMP reductase molecular species, which can be used to explain the bimodal saturation curve noted with the purified human erythrocyte GMP reductase.