[The nitric oxide cycle in mammals and nitrite reducing activity of heme-containing proteins]. 
A nitric oxide cycle is proposed, which explains numerous experimental data demonstrating that under hypoxic conditions synthesis of NO was elevated.  Two pathways appear to be fundamental for activation of NO formation: 1) activation of L-arginine utilization as its content was decreased in blood; 2) transfer of heme-containing proteins hemoglobin, myoglobin, cytochrome oxidase and cytochrome P-450 into their deoxy-form, where these proteins are able to reduce NO2- into NO.  This suggests that the nitric oxide cycle may be considered as a compensatory mechanism which allowed the various cells to acquire reduced dependence on overloading under conditions of oxygen and energy deficiency.