Effect of manganese on the hepatic microsomal mixed function oxidase enzyme system in the rat

Experiments were conducted to examine the effect of manganese on the hepatic mixed function oxidase system in the rat. Acute treatment with manganese chloride (1-10 mg Mn/kg, ip) produced a significant prolongation of hexobarbital hypnosis in male rats on Days 2 and 3 following metal administration. The threshold dose of manganese to produce this alteration in response was 5 mg Mn/kg and the altered response returned to control values by Day 5. The prolonged hexobarbital hypnosis resulted from Mn inhibition of the hepatic microsomal mixed function oxidase system, the activity of which was assessed using aniline (23%), ethylmorphine (26%), and hexobarbital (27%) as substrates. Manganese treatment also produced significantly reduced levels of cytochrome P-450 (23%) and b5 (21%), but the substrate-induced spectral binding of all three substrates was not altered significantly by Mn when expressed as delta A per nanomole of cytochrome P-450. The activity of NADPH cytochrome c reductase was also significantly decreased (25%) by Mn treatment. Following the in vitro addition of Mn in concentrations ranging from 1 X 10(-6) to 1 X 10(-3) M Mn to microsomes derived from naive rats, there was no decrease in the metabolism of aniline or hexobarbital or cytochrome P-450 levels. Significant inhibition in ethylmorphine metabolism was observed with Mn concentrations of 1 X 10(-4) M and greater. These experiments indicate that acute Mn treatment can alter drug response as the result of decreased hepatic biotransformation which occurs by an indirect mechanism.