isoenzymen.

1960 New Eng. Jrnl. Med. 15 Sept. 531 (heading) Isoenzymes and myocardial infarction.

1964 Oceanogr. & Marine Biol. 2 220 Species- and tissue-specific forms of enzymes (iso-enzymes) have been reported in animal material.

1968 A. L. Latner & A. W. Skillen Isoenzymes in Biol. & Med. i. 1 It is now well recognized that a large number of enzymes exist in multiple forms. This applies not only to tissues and tissue extracts but also to enzyme proteins which have been isolated in the crystalline state and are really mixtures. Isoenzymes are examples of these multiple forms. Precise definition of the word ‘isoenzyme’ is, however, rather difficult. Different tissues of the same individual or even of different species may possess closely similar enzymes, which are not really isoenzymes. For the time being, most authorities believe that a broad definition such as ‘different proteins with similar enzymatic activity’ best suits the current state of our knowledge. It is customary, for the most part, to limit this definition to multiple enzymes obtained from one tissue of one individual animal or plant or possibly a small organ, or a culture of a unicellular organism. An exception would be the major multiple forms of human alkaline phosphatase. They are nevertheless referred to as isoenzymes.

1970 Nature 30 May 862/1 The enzyme glucose-6-phosphate dehydrogenase..exists in several different forms (isoenzymes) in the human erythrocyte... The structural differences between the isoenzymes lead to different enzymatic activities.

1968 Brit. Med. Bull. 24 222/2 Chemical differences between individuals have been much studied..for example..the occurrence of isoenzymic variants of a number of well-known enzymes.