Peroxisomes aminotransferases

Hyperoxaluria type 1 is due to functional efficiency of the liver specific peroxisomal enzyme alanine-glyoxylate aminotransferase. It leads to severe renal failure. The nervous system is not affected. 

Located in the peroxisomes of liver, L-alanine-glyoxylate aminotransferase catalyzes the transamination of alanine and glyoxylate to form pyruvate and glycine. A rare inborn error of metabolism manifested as hyperoxaluria is due to a deficiency of this enzyme. 

Hepatitis describes infiltration of the hepatic tissue by mononuclear cells, which may or may not be associated with hepatocellular changes. There are also different patterns of cellular injury, such as those affecting the hepatocellular organelles— particularly, the microsomes, peroxisomes, and mitochondria. When hepatocellular injury occurs, the aminotransferases are probably the most useful markers, and they may be supplemented by measurements of other plasma enzymes—ALP, GLDH, and plasma bilirubin (see the following sections on laboratory investigations). Several markers of cellular function require tissues for the measurements of altered function (e.g., microsomal cytochrome P450 measurements). 

Kinetic data are available for many aminotransferases, though in most cases the enzyme preparations have consisted either of crude extracts or incompletely purified proteins. Examples of such data are shown in Table 1. The An, value for the keto acid substrate is usually lower than the for the amino acid substrate, but there are exceptions to this generalization. Observed An, values may depend on the conditions of assay, and especially on the ionic composition of the buffer employed. For animal enzymes the anionic components of the buffer are said to be of particular importance in affecting kinetic parameters (cf. Braunstein, 1973). Plant enzymes have not been very systematically investigated in this respect, but phosphate inhibition has been observed for a peroxisomal enzyme transaminating with glyoxylate (Rehfeld and Tolbert, 1972). 

The enzymes utilizing serine as amino donor and glyoxylate as amino acceptor are considered together here. Much attention has been paid to these aminotransferases, with the role of these enzymes in photorespiration being a particular focus of interest (see Keys, this volume. Chapter 9). Initial studies on impure extracts indicated the presence of glyoxylate aminotransferases which could use glutamate, aspartate, alanine, and serine as amino donors. The best donor to glyoxylate varied from one tissue to another (Cossins and Sinha, 1965). Leaf peroxisomes were subsequently shown to... 

Density gradient fractionation techniques some time ago demonstrated the presence of glutamateiglyoxylate aminotransferase in leaf peroxisomes (Kisaki and Tolbert, 1%9). Similar methods subsequently confirmed the presence of peroxisome-localized serinerpyruvate and aspartate aminotransferases in spinach leaves (Yamazaki and Tolbert, 1970). The same study demonstrated aspartate aminotransferase in both chloroplasts and mitochondria. A microbody location for aspartate aminotransferase was also observed by Cooper and Beevers (1969), who purified castor bean glyoxy-somes on density gradients. 

Gel electrophoresis evidence suggested that different aspartate aminotransferase isoenzymes were present in chloroplasts and mitochondria from spinach leaves. In addition, two electrophoretically distinct forms of aspartate aminotransferase were seen in the peroxisomal fraction (Yamazaki and Tolbert, 1970). Kanamori and Matsumoto (1974) found two isoenzymes of glutamate oxaloacetate aminotransferase in roots of rice seedlings, whereas there were three isoenzymes in the shoots. However, in the roots, where the soluble and mitochondrial enzymes both consisted of multiple forms, the electrophoretic pattern was similar for both, i.e., the electrophoretic forms were not organelle specific. Other workers concluded that extensively purified mitochondrial and soluble alanine aminotransferases in tomato fruits were the same protein (Gazeu-Reyjal and Crouzet, 1976) and that there were not organelle-specific isoenzymes. 

In leaves glycolate oxidase is restricted to the peroxisomes (Tolbert, 1971) which also contain high catalase and aminotransferase activity. This presumably favors conversion of the glyoxylate to glycine and means that little formate is produced in vivo. 

Leiper JM, Oatey PB, Danpure CJ Inhibition of alanine glyoxylate aminotransferase 1 dimerization is a prerequisite for its peroxisome-to-mitochondrion mistargeting in primary hyperoxaluria type 1. J Cell Biol 1996 135 939-951. 

Primary hyperoxaluria type I (PH I) (Fig. 20.2) is a rare, autosomal recessive inherited disease caused by a defect in glyoxylate metabolism with low or absent activity of liver-specific peroxisomal ala-nine-glyoxylate aminotransferase (ACT) (Danpure 1989). The AGXT gene is located on chromosome 2q36-37 (Purdue et al. 1991). The disease prevalence is two patients per million population (Kopp and Leumann 1995) in Europe. 

Somerville and Ogren (1980b) initially isolated a mutant of A. thaliana lacking peroxisomal serine glyoxylate aminotransferase. The mutant was detected by demonstrating that C02 accumulated in glycine and serine following photosynthesis in air. Similar mutant lines have also been detected in barley (Murray et al, 1987) and tobacco (Havir and McHale, 1988). 

In 1980 (Lea and Miflin, 1980) the only well-documented report of the enzyme activity was in soybean leaves (Streeter, 1977), although Uoyd and Joy (1978) had detected the conversion of [ ]asparagine to [ ]hydroxysucdna-mic acid in pea leaves, probably using 2-oxosuccinamic acid as an intermediate (see steps 1 and 2 in Fig. 6). Asparagine aminotransferase activity has been detected in high levels in leaves (where it is present in the peroxisomes see Ireland and Joy, 1983a), but the enzyme is either absent or present only in very... 

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