The guanosine monophosphate reductase gene is conserved in rats and its expression increases rapidly in brown adipose tissue during cold exposure.

Non-shivering thermogenesis is required for survival of rodents during cold stress. Uncoupling protein-1 acts in brown adipose tissue (BAT) to transport protons, thus dissipating the proton gradient across the inner mitochondrial membrane. This permits respiration uncoupled from ATP synthesis. UCP-1 function is inhibited by the binding of purine nucleotides, with GTP/GDP being more potent than ATP/ADP. We used a cDNA subtraction analysis to identify cDNAs rapidly induced by cold exposure. One of these encodes rat guanosine monophosphate reductase (GMP-r). This was surprising in that previous data had suggested that this enzyme was absent in rodents. Rat GMP-r is 96% identical to human GMP-r, and its mRNA is increased 30-fold in BAT within 6 h of cold exposure. The gene is also expressed (but not cold-responsive) in muscle and kidney, but not in white fat. We speculate that the physiological function of the marked increase in BAT GMP-r during cold stress may be to deplete the brown adipocyte of guanine nucleotides, converting them to IMP, thus permitting enhanced UCP-1 function. This is a previously unrecognized regulatory aspect of thermogenesis, an essential physiological response of rodents to cold.