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Several biochemical parameters reflecting the degree of senescence were compared between senile-prone (P) and resistant (R) strains of male senescence-accelerated mouse (SAM) at 11 to 12 months of age. Plasma testosterone in SAM-P amounted to half the concentration in SAM-R. In the liver and brain of SAM-P, the content of malondialdehyde (MDA) and the activity of monoamine oxidase B (MAO-B) were significantly higher than those in SAM-R. Moreover, large amounts of fluorescent lipofuscin were detected in the SAM-P liver. Compared with SAM-R, both membrane and cytosolic fractions from the SAM-P liver contained small amounts of protein and showed less activity of superoxide dismutase (SOD). The present findings suggest that the male SAM-P rapidly acquires biochemical changes which are considered to be part of the normal aging process.

Hypoxanthine was demonstrated to have a dose-dependent inhibitory effect on type B monoamine oxidase (MAO-B) activity in liver and brain tissues, and slight inhibitory effect on type A MAO(MAO-A) activity when given orally to mice at doses of 25-500 mg/kg. When mice were given orally hypoxanthine 500 mg/kg, MAO-A and -B activities were all inhibited significantly 16 hours after administration, but the inhibitory action on MAO-A was weaker. Subcutaneous injection of the agent also produced obvious inhibition of MAO activity in the liver but no significant influence on MAO activity in brain tissue was observed. In vitro experiment showed that the action of hypoxanthine on MAO-B was competitive inhibition and that on MAO-A was competitive mixed with noncompetitive inhibition.