Isoenzyme, definition

The multienzymic nature of pyruvate kinase has been investigated in detail in M. expansa (98), H. diminuta (117) and S. erinacei (240). These worms possess FBP-sensitive and FBP-insensitive pyruvate kinase isoenzymes. In H. diminuta, as many as five pyruvate kinase isoenzymes (for definition and usefulness, see Chapter 6) occur during development (Fig. 5.3) and it seems likely that differential expression of these different forms of the enzyme may help to control the specific composition of excreted end-products by the various life cycle stages. The nature and regulation of the end-products secreted in H. diminuta are discussed further below. 

L-(-f-)-Tartrate (0.05 M) inhibited isoenzymes Nos. 1-4 but had no appreciable effect on the reticular cell isoenzyme. No. 5 (M8, Yl). In cytochemical studies of blood smears from three patients with leukemic reticuloendotheliosis, the acid phosphatase activity in the monocytes, eosinophiles, neutrophiles, and other cells that could definitely be identified as lymphocytes did not differ appreciably from those of normal subjects. In all three patients the neoplastic reticulum cell showed various degrees of acid phosphatase activity most of them were strongly positive. The enzyme activity in these cells was resistant to to l-( + )-tartrate, whereas it was completely inhibited in other types of cells. 

Are these so-called organelle-specific isoenzymes really isoenzymes in the true sense Comparative arguments based on the existence of true organelle-specific isoenzymes in animals seem persuasive. However, it must be emphasized again that here, as in so many other aspects of the biochemistry of plant aminotransferases, definite conclusions must wait until more homogeneous enzyme preparations are available for study. 

No proof has been obtained that a 3//-indolylidene (or the corresponding cation) is an intermediate in the oxidative cyclization, and it must be concluded that the substrate remains bound to the same site on the enzyme molecule throughout the whole process. It can be expected that a (reactive) compound similar in structure to the proposed intermediate will not be released from the enzyme after the dehydrogenation reaction. Further elucidation of the mechanism of the conversion of pCA into aCA requires more definite evidence that the cyclization step is also catalyzed by the enzyme. Efforts to obtain such evidence are complicated by the fact that the isoenzymes do not catalyze the reverse reaction namely, the conversion of aCA into pCA (Schabort and Potgieter, 1971). 

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