PURIFICATION AND PARTIAL CHARACTERIZATION OF 3-HYDROXYISOBUTYRYL-COENZYME-A HYDROLASE OF RAT-LIVER

An unusual feature of valine catabolism is a reaction in which an inte rmediate of its catabolic pathway, (S)- 3-hydroxyisobutyryl-CoA, is hy drolyzed to give the free acid and CoA-SH. The enzyme responsible for this reaction, 3-hydroxyisobutyryl-CoA hydrolase (EC 3.1.2.4), was pur ified 7200-fold from rat liver in this study. The purified enzyme cons ists of a single polypeptide with an M(r) of 36,000 in the native and denatured forms. The hydrolase is highly specific for (S)-3-hydroxyiso butyryl-CoA and 3 hydroxypropionyl-CoA (K-m, 6 and 25 mu M, respective ly) with optimal activity around pH 8. The turnover rate of the enzyme for (S)-3-hydroxyisobutyryl-CoA is 270 s(-1), which is high relative to other enzymes of the valine pathway. Likewise, activity of the enzy me expressed on a wet weight basis is also very high in the major tiss ues of the rat. These findings suggest that rapid destruction of (S)-3 -hydroxyisobutyryl-CoA produced during valine catabolism is physiologi cally important. We propose that the need for a mechanism to protect c ells against the toxic effects of methacrylyl-CoA which is maintained in equilibrium with (S)-3-hydroxyisobutyryl-CoA by crotonase, explains why valine catabolism involves this enzyme and why its tissue activit y is so high.