The protein part of an enzyme without the cofactor necessary for catalysis. The cofactor can be a metal ion, an organic molecule ( coenzyme ), or a combination of both.
PAC, 1994, 66, 2587 (Glossary of bioanalytical nomenclature - Part 1: General terminology, body fluids, enzymology, immunology (IUPAC Recommendations 1994)) on page 2593