Table 1. A test of the hypothesis that unbound NADPH displaces tightly bound NADP⁺ during the prevention of the formation of compound II.

For further details, see the Experimental section. Catalase was exposed, at 37 °C, to [¹⁴C]NADPH that was kept constantly in a reduced state by the presence of yeast G6P dehydrogenase and G6P. Each reaction mixture contained glucose oxidase (1.6 μg·ml⁻¹). H₂O₂ was generated in the indicated mixtures by the addition of glucose. At the indicated intervals, aliquots were withdrawn for measurement of 6-phosphogluconate concentration and catalase-bound [¹⁴C]NADPH. The amount of NADPH oxidized (n) was considered to be the amount of 6-phosphogluconate generated. The reactions were stopped by placing in a boiling water bath; incubation times therefore were slightly greater than indicated. Difference=(with H₂O₂−without H₂O₂) for each case.

	sb (nmol of [14C]NADPH/mol of catalase)			n (6-phosphogluconate) (nmol·ml−1)			Displacement (%)
Incubation time (min)	+H2O2	−H2O2	Difference	+H2O2	−H2O2	Difference	Expected†	Observed‡
0	0.018	0.017	0.001	8.1	1.1	7.0	98.7	0.6
30	0.050	0.048	0.002	19.4	2.1	17.3	100.0	1.0
60	0.048	0.046	0.002	26.4	1.8	24.6	100.0	1.2
120	0.049	0.046	0.004	58.9*	3.0	55.9	100.0	2.0

* Less than 0.05 nmol·ml⁻¹ of 6-phosphogluconate was generated in 120 min when G6P dehydrogenase was omitted from this incubation mixture.

† From eqn 6 (see the Experimental section) and the values for difference for n. r=0.625 nmol⁻¹ and S_m=30370 c.p.m. nmol⁻¹.

‡ From eqn 6 and the values for difference for s_b.