Table 1.

Kinetics of arylhydroxylamine carcinogen reduction by human liver microsomes or purified soluble NADH cytochrome b₅ reductase (b5R) and cytochrome b₅ (cyt b5).^a

Substrate	Enzyme Source	Km (μM)	Vmax (nmol/mg protein/min)
NHOH-4-ABP	HLM	197 ± 1 b	4.13 ± 0.05
	b5R/cyt b5	220 ± 89 b	233 ± 36 d
NHOH-PhIP	HLM	229 ± 11 c	1.57 ± 0.01
	b5R/cyt b5	225 c	544 e

The maximal velocity (V_max) of arylhydroxylamine carcinogen reduction by purified b5R/cyt b5 was 56-fold

and 346-fold

higher than in HLM for NHOH-4-ABP and NHOH-PhIP, respectively. All reactions were performed in PBS, pH 7.4, with 1 mM NADH and 3 mM ascorbate, under conditions approximating linear kinetics. In the purified system, b5R and cyt b5 were used at a 1:10 stoichiometry. Velocity data are expressed in nmol per total mg protein per min. Data represent two to three experiments performed in duplicate for each condition, except for NHOH-PhIP by b5R/cyt b5, for which data are reported for one experiment performed in duplicate. Results are given as mean ± SD.

^aK_m values were not significantly different between HLM and b5R/cyt b5 for either substrate

^bP = 0.75;

^cP = 0.65, one sample t test.

^dP = 0.01

^eP < 0.0001; one sample t test