The effects of the non-ionic detergent Tween 80 on hepatic microsomal hydroxylation.

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 = 0-47 mM, 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; deltaEmax = 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.