Hydrolyzes 3-hydroxyisobutyryl-CoA (HIBYL-CoA), a saline catabolite. Has high activity toward isobutyryl-CoA. Could be an isobutyryl-CoA dehydrogenase that functions in valine catabolism. Also hydrolyzes 3-hydroxypropanoyl-CoA (By similarity). 3-hydroxy-2-methylpropanoyl-CoA + H2O = 3-hydroxy-2-methylpropanoate + CoA + H(+) Optimum pH is 7-9 with 3-hydroxyisobutyryl-CoA as substrate and 6 with 3-hydroxypropionyl-CoA as substrate. Amino-acid degradation; L-valine degradation. Highest activity in liver, kidney and heart. Low activity in muscle and brain. Belongs to the enoyl-CoA hydratase/isomerase family.