IUPAC‐IUB Commission on Biochemical Nomenclature (CBN)

Recommendations on the nomenclature of multiple forms of enzymes were prepared by a subcommittee appointed by the International Union of Biochemistry and published in 1964 in a number of journals (1).These were later revised by a subcommittee of the IUPAC-IUB Commission on Biochemical Nomenclature (CBN) and the revised recommendations were published in 1971 (2).In view of the rapid development in the field, particularly of conjugated or derived forms of enzymes as related to enzyme regulation, a new subcommittee1 was appointed to unify the nomenclature of covalently modified enzymes.The report of the subcommittee was received by CBN in 1972 and circulated widely among experts in the field.The recommendations of the subcommittee,' including suggestions received by CBN, were incorporated by CBN into the present set of recommendations covering all multiple forms of enzymes.These recommendations were adopted in 1976. I. Definition of lsozymes or IsoenzymesThe 1964 Committee recommended (1) that "multiple enzyme forms" in a single species should be known as isoenzymes (or isozymes).It is known that enzymes catalyzing essentially the same reaction may differ in various ways, as shown in Table I.In this table, some prominent examples are alsoAccording to the original definition, which was meant to be purely operational, all these multiple forms should be termed "isozymes" or "isoenzymes."However, many biochemists feel that the term "isoenzymes" should be restricted to those forms arising from genetic control of primary protein structure.Genetically determined differences in primary structure are the reason for the multiplicity in Groups 1 to 3 of Table I, but not in Groups 4 to 6. Indeed, investigators working on conjugated or derived enzymes do not use the isozyme terminology for characterization of their multiple forms.It is therefore recommended that:1.The term "multiple forms of the enzyme . .." should be used as a broad term covering all proteins catalyzing the same reaction and occurring naturally in a single species.2. The term "isoenzyme" or "isozyme" should apply only to