Unusual pattern of beta-phenylethylamine deamination in the rat heart

The present study was aimed to determine type A and B MAO activities in rat heart and renal cortex homogenates and evaluate the sensitivity of deamination of 3H-5-HT and 14C-beta-PEA to selective MAO-A and MAO-B inhibitors, respectively Ro 41-1049 and lazabemide. Deamination of beta-PEA in the rat heart was not affected (Vmax = 53+/-10 vs 42+/-6 nmol mg protein(-1) h(-1)) by lazabemide (250 nM), but was significantly reduced (Vmax = 10+/-1 nmol mg protein(-1) h(-1)) by Ro 41-1049 (250 nM). Deamination of beta-PEA in the rat heart is a low affinity process (when compared with that in the kidney) with high Km values (244+/-98 vs 18.6+/-5.8 microM). On the other hand, deamination of 5-HT in the rat heart and renal cortex revealed high Km values, which were similar to those for beta-PEA in the heart. Deamination of beta-PEA (1000 microM) in the rat heart was inhibited in a concentration-dependent manner by Ro 41-1049 with a Ki value of 32 nM (22, 48; 95% confidence limits), but not by the selective MAO-B inhibitor lazabemide (up to 500 nM). Inhibition of 5-HT (1000 microM) deamination in the rat heart by Ro 41-1049 was also a concentration-dependent process with a Ki value of 21 (16, 26) nM. Deamination of 5-HT (1000 microM) in the rat renal cortex, was inhibited in a concentration-dependent manner by Ro 41-1049 with a Ki value of 12 (8, 17) nM. Deamination of beta-PEA in the renal cortex was inhibited by lazabemide with a Ki of 5 (3, 7) nM. In the rat heart, in contrast to that in the renal cortex, the specific MAO-B substrate beta-PEA is deaminated by a form of MAO which most probably corresponds to MAO-A.