Isozymes general

GSH-S deficiency is a more frequent cause of GSH deficiency (HI7), and more than 20 families with this enzyme deficiency have been reported since the first report by Oort et al. (05). There are two distinct types of GSH-S deficiency with different clinical pictures. In the red blood cell type, the enzyme defect is limited to red blood cells and the only clinical presentation is mild hemolysis. In the generalized type, the deficiency is also found in tissues other than red blood cells, and the patients show not only chronic hemolytic anemia but also metabolic acidosis with marked 5-oxoprolinuria and neurologic manifestations including mental retardation. The precise mechanism of these two different phenotypes remains to be elucidated, because the existence of tissue-specific isozymes is not clear. Seven mutations at the GSH-S locus on six alleles—four missense mutations, two deletions, and one splice site mutation—have been identified (S14). 

The ACS function is known only in higher plants. The activity of ACS isozymes is a key regulatory factor of ethylene biosynthesis pathway. In general, microorganisms liberate ethylene but their ethylene synthesis pathways do not involve ACC as an intermediate. Penicillium citrinum is the first reported microorganism that is able to synthesize ACC from SAM and to degrade it into ammonia and a-ketobutyrate, not to ethylene. ACS from P. citrinum shows a 100-fold higher for SAM than its plant counterparts. ... 

It is generally accepted that chloroplasts possess an intact pathway of aromatic amino acid biosynthesis that is tightly regulated. In addition, the subcellular location of some aromatic-pathway isozymes has been shown to be in the cytosol, but whether an intact pathway exists in the cytosol has not yet been proven. The evidence bearing on aromatic amino acid compartmentation and regulation is reviewed, with particular emphasis given to the relationship between primary biosynthesis and secondary metabolism in the cytosol. 

The only biological function which has been repeatedly confirmed is the role of peroxidases in lignin monomer polymerization (1). But even in this case, the role of the various isozymes is not yet clear, although anionic, cell wall bound peroxidases generally seem to be involved in lignification... 

It should be noted that this is not an exhaustive list and that in different species, different numbers of isozymes exist. It serves solely to illustrate the multiplicity of the forms of cytochrome P-450 generally involved with xenobiotic metabolism and the differences and similarities between them. 

The four forms of hexokinase found in mammalian tissues are but one example of a common biological situation the same reaction catalyzed by two or more different molecular forms of an enzyme. These multiple forms, called isozymes or isoenzymes, may occur in the same species, in the same tissue, or even in the same cell. The different forms of the enzyme generally differ in kinetic or regulatory properties, in the cofactor they use (NADH or NADPH for dehydrogenase isozymes, for example), or in their subcellular distribution (soluble or membrane-bound). Isozymes may have similar, but not identical, amino acid sequences, and in many cases they clearly share a common evolutionary origin. 

In general, the distribution of different isozymes of a given enzyme reflects at least four factors ... 

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