Intracellular oxygen supply during hypoxia.

The intracellular supply of O2 to mitochondria was studied in single-cell suspensions of rat hepatocytes by measuring the O2 dependence of oxidation of cytochromes. Values were obtained by adding standardized O2-containing solutions to anaerobic cell suspensions and observing absorbance changes at wavelength pairs specific for cytochromes a + a3, c + c1, and b561 + b566. Half-maximal oxidation, calculated relative to aerobic cells, occurred at 3.5, 6.2, and 4.9 microM O2, respectively. These values are similar to those for changes in cellular ATP/ADP, lactate/pyruvate, and NADH/NAD+ but are much higher than corresponding values for isolated mitochondria. Analysis of these data indicate that the diffusion coefficient in the region of mitochondria in situ is considerably smaller than the extracellular diffusion coefficient and suggests that a significant O2 gradient in the vicinity of mitochondria occurs under hypoxic conditions. These results suggest that the rate of O2 diffusion may be a critical factor for intracellular O2 supply to mitochondria during hypoxia.