Age (juvenile to aged) and sex effects on in vitro CYP2A6 enzyme activity in African green monkeys using nicotine and coumarin

CYP2A6 is the principal human enzyme metabolizing coumarin and nicotine; it demonstrates wide variation from genetics and environmental regulation, complicating the determination of human developmental profile and sex-related differences. African green monkeys (AGMs), with comparable CYP2A6 activity to humans, of both sexes across 5 developmental stages (N = 56) were used to investigate age and sex effects on CYP2A6 activity using 7-hydroxycoumarin formation from coumarin, and cotinine formation from nicotine. Age and sex differences were determined using two-way ANOVAs and post hoc tests. Age impacted microsomal protein per gram liver (determined by cytochrome P450 oxidoreductase activity). Age and sex differences, with interactions, were observed in liver and body weight, whereby older AGMs and males were heavier for both. Pooled microsomes per age-sex group were used to determine the kinetic parameters for 7-hydroxycoumarin (overall Km = 1.5 ± 0.3 μM, Vmax = 0.34 ± 0.06 nmol/min/mg) and cotinine (overall Km = 37.4 ± 8.0 μM, Vmax = 1.36 ± 0.23 nmol/min/mg) formation. Age impacted the velocity of 7-hydroxycoumarin and cotinine formation (individual AGMs assessed at substrate concentrations at ∼Km and ∼10 × Km), at per milligram of microsomal protein, per gram liver, and per whole liver normalized by body weight, whereby juveniles and adolescents had higher velocities. Both age and sex impacted 7-hydroxycoumarin and cotinine formation per whole liver, but in contrast, older AGMs and males had higher velocities. In conclusion, age but not sex impacted CYP2A6 activity, which was higher in younger AGMs for essentially all denominators except following extrapolation to the whole liver, where activity was influenced by larger livers in adults and males. SIGNIFICANCE STATEMENT: Microsomal CYP2A6 activity was higher in younger African green monkeys (AGMs), with no sex differences. Once extrapolated to the whole liver, representing the total rate of metabolism, the velocity was faster in older AGMs and males, primarily due to age and sex differences in liver weight. There were significant age differences in CYP2A6 activity, where younger AGMs had higher velocity per milligram-microsomal protein, gram liver, and whole liver normalized to body weight levels, while older and male AGMs had higher whole liver velocity.