In order to clarify the effect of cytochrome b
5 on Δ
16-C
19-steroid synthesizing activity in the testicular microsomes of neonatal pig, the effects of antibody and pyridine nucleotide on cytochrome b
5 were investigated. Furthermore, the relationship between Δ
16-C
19-steroid synthesizing activity and cytochrome b
5 was studied by use of adrenal microsomes or a reconstituted enzyme system of cytochrome P-450 (17α-hydroxylase/lyase) purified from adrenal microsomes. With anti-cytochrome b
5 IgG no remarkable change was observed in the 17α-hydroxylase/lyase activity of testicular microsomes. Δ
16-C
19-Steroid synthesizing activity was, however, strongly inhibited. In the testicular microsomes or the reconstituted enzyme system of cytochrome P-450 (17α-hydroxylase/lyase) isolated from testicular microsomes, synergetic effects of nicotinamide adenine dinucleotide phosphate and nicotinamide adenine dinucleotide against the Δ
16-C-
19-steroid synthesizing activity and 17α-hydroxylase/lyase activity was observed. A strong effect was detected on the Δ
16-C
19-steroid synthesizing activity, while this synergetic effect against 17α-hydroxylase/lyase activity was weak. Although in adrenal microsomes little Δ
16-C
19-steroid synthesizing activity was detected, this activity was restored by the addition of cytochrome b
5 isolated from testicular microsomes. Similarly to microsomes, Δ
16-C
19-steroid activity was restored by the addition of cytochrome b
5 to the reconstituted enzyme system of cytochrome P-450 (17-hydroxylase/lyase) isolated from adrenal microsomes.
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