Table 1.

Semi-quantitative descriptors of ‘hypoxia’.

pO2 (mm of Hg)	Description	% Max. EF5 Metab	Abs Bind Rate (aM/cell/hr)
76	Physiological	1	1.5
19	Mild Hypoxia	3	5
3.8	Moderate Hypoxia	10	15
0.76	Severe Hypoxia	30	50
0.0	Anoxia	100	150

In order to calculate the absolute binding rate (column 4) cells were incubated at various oxygen levels in the presence of 100 µM C14-labelled EF5 and after various times, the cells were rinsed thoroughly, and treated with 5% TCA to allow complete disruption with precipitation of macromolecules (26). The total binding rate (shown) consists of about 25% acid insoluble and 75% acid soluble metabolites. The rates shown are for the V79 Chinese hamster cells, which are similar to the average of several other cell lines (49). Since these cells have a mass of about 10^–12 kg, the rate under anoxia implies that the amount of metabolized drug is about 1.5 times the extracellular drug concentration, per hr.