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Abstract
1. Effects of the non-ionic detergent Tween 80 on hamster liver microsomal components and on various reactions, including biphenyl hydroxylation, have been studied. Tween 80 competitively inhibited both 2- and 4-hydroxylation of biphenyl, with Ki values of 4·9 and 4·1 mM. The apparent kinetic constants corrected for the inhibitory effect of 2·9 mM Tween 80 (the concn. routinely used) were for 2-hydroxylation: Km = 0·47mM, Vmax = 2·2 nmol min/mg; and for 4-hydroxylation: Km= 0·14 mM, Vmax = 3·5 nmol/min/mg.
2. The type I spectrally apparent interaction of biphenyl with liver microsomes (Ks = 0·23 mM; δEmax = 8·2 E/2 mg protein) was also competitively inhibited by Tween 80 (Ki = 1·1 mM), itself an apparently type I substrate.
3. Tween 80 (9·5 mM) activated aniline 4-hydroxylation. The type II spectrally obvious interaction between aniline and microsomes was not affected by Tween 80.
4. Tween 80 and biphenyl together enhanced NADPH-cytochrome c reduction whereas Tween 80 and aniline together had no effect. Tween 80 did not affect the stability of cytochrome P—450 in microsomal suspensions over a 5 min period.
5. Results are discussed in terms of differences between biphenyl 2- and 4-hydroxylation systems, between type I and II substrate-enzyme complexes, and in terms of the relevance of spectral interactions to aryl hydroxylations.