High affinity binding of [ 3H] cocaine to rat liver microsomes

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[ 3H]Cocaine bound reversibly, with high affinity (K D 2.3 ± 1.1. nM) and stereospecificity to rat liver microsomes. Little binding was detected in the lysosomal, mitochondrial and nuclear fractions. The binding kinetics were slow ( T 1 2 for association, 6 min and for dissociation 17 min), and the kinetically calculated K D was 2 nM. Induction of mixed function oxidases by phenobarbital did not produce significant change in [ 3H] cocaine binding. On the other hand, chronic administration of cocaine reduced [ 3H] cocaine binding drastically. Neither treatment affected the affinity of the liver binding protein for cocaine. Microsomes from mouse and human livers had less cocaine-binding protein and lower affinity for cocaine than those from rat liver. Binding of [ 3H] cocaine for rat liver microsomes was insensitive to monovalent cations and > 10 fold less sensitive to biogenic amines than the cocaine receptor in rat striatum. However, the liver protein had higher affinity for cocaine and metabolites except for norcocaine. Amine uptake inhibitors displaced [ 3H] cocaine binding to liver with a different rank order of potency than their displacement of [ 3H] cocaine binding to striatum. This high affinity [ 3H] cocaine binding protein in liver is not likely to be a monooxygenase, but may have a role in cocaine-induced hepatotoxicity.