The cellular location of catechol-O-methyltransferase in rat liver

Catechol-O-methyltransferase (COMT) activity on the extracellular face of the plasma membrane of isolated rat hepatocytes was assayed, and 4.3% of total COMT activity was located there in cells which satisfied our criteria of viability. However, since 1.2% of the cells' lactate dehydrogenase activity was also apparently extracellular, and this proportion increased to 3.4% under the conditions of the COMT assay the amounts of extracellular COMT may be even less. COMT in rat liver microsomes and plasma membranes represent 2.3% and 0.08% of total rat liver COMT respectively. This implies an insignificant role for plasma membrane COMT although reported altered kinetic behaviour could elevate microsomal COMT to a supporting role in the regulation of catecholamine concentration in the circulation. Since by far the largest fraction of COMT is located intracellularly in the soluble cell fraction, the physiological functions of COMT seem to be dependent on the passage of substrates through the cell membrane for their presentation to the enzyme.