Catechol-O-methyltransferase from rat liver: two forms having different meta:para methylation ratios.

Cytoplasmic catechol-O-methyltransferase activity from rat liver was resolved by gel filtration into two enzymes: a major form having an estimated molecular weight of 23,000 and a minor one of 45,500. The relative abundance of these forms in liver is about 5:1, respectively. Microsomal catechol-O-methyltransferase constituted only 2% of the total liver activity. After solubilization by sonication most of the microsomal enzyme showed a molecular weight in excess of 100,000, but some 23,000 - enzyme was also released. The bound enzyme thus may represent an aggregate form of the soluble activity. The two cytoplasmic enzymes differ in several properties, including pH optima and thermal stability. The two forms also differ in the extent of methylation of the $\text{para}$ hydroxyl group, the larger enzyme having a $\text{meta}$:$\text{para}$ methylation ratio twice that obtained with the smaller form.