Production of ornithine by intact human erythrocytes

Although the conversion of arginine to ornithine in humans is largely an hepatic process, erythrocytes also have this enzymatic capacity. We have shown that intact human erythrocytes readily produce ornithine from arginine and release product ornithine to the incubation medium. The rate of ornithine formation from arginine was 0.22 mumol.h-1.ml cells-1, and 85% of the ornithine is recovered extracellularly. Moreover, we have shown that ornithine and urea are the unique and stoichiometric products of arginine metabolism in the intact erythrocyte. The rate of ornithine production by intact red blood cells was a saturable function of arginine concentration in the medium; the derived Km for arginine for this conversion was 0.16 mM, a value striking for its close approximation to physiological arginine concentration in human plasma. We propose that the production and delivery of ornithine by intact red blood cells may supply peripheral tissues such as bone and muscle with an important precursor for proline and polyamines.