Galactose-l-phosphate uridyl transferase

Enzymes, measured in clinical laboratories, for which kits are available include y-glutamyl transferase (GGT), alanine transferase [9000-86-6] (ALT), aldolase, a-amylase [9000-90-2] aspartate aminotransferase [9000-97-9], creatine kinase and its isoenzymes, galactose-l-phosphate uridyl transferase, Hpase, malate dehydrogenase [9001 -64-3], 5 -nucleotidase, phosphohexose isomerase, and pymvate kinase [9001-59-6]. One example is the measurement of aspartate aminotransferase, where the reaction is followed by monitoring the loss of NADH ... 

Reaction (1) is irreversible the enzyme galactokinase has been found in mammalian liver, brain, and erythrocytes, as well as in certain yeasts and other microorganisms. UDPGal, the main product of reaction (2), is the intermediate by means of which the body incorporates galactose into cerebrosides and, probably, other galactolipids, mucopolysaccharides, and lactose galactose-l-phosphate uridyl transferase occurs in the liver, to a lesser extent in red cells, and probably in other tissues. 

Gal-l-P + UDPG = G-l-P + UDPGal Galactose-l-phosphate uridyl transferase... 

The galactose-l-phosphate uridyl transferase pathway is by far the most important in galactose metabolism it accounts for about 5/6 of the total galactose metabolized in mammals. It is less active in the newborn than in the adult, e.g., the liver of newborn rats has only 1/5 the adult amount of galactose-l-phosphate uridyl transferase per milligram of protein (13). 

A similar reaction had previously been known to occur in Saccharomyces fragilis (K8) and in bean seedlings (N3). This is an alternative route to UDPGal, but the activity of the enzyme in newborn liver is very low even in the adult it is only one-sixth that of galactose-l-phosphate uridyl transferase (13), as shown in Table 2. 

All the varied biochemical and clinical features of galactosemia are, beyond any reasonable doubt, caused by the absence of this one enzyme, galactose-l-phosphate uridyl transferase. Yet it is not possible, at present, to identify all the steps by which the different effects are brought about. 

Improvements in these methods are likely to take the form of improved specificity for galactose-l-phosphate uridyl transferase and improvements in preparing the red cell hemolyzates. Sensitivity and precision of the assays are now good enough to reveal a puzzling feature some galactosemics appear to have a very small amount of enzymatic activity in their red cells. This is discussed further in Section 5. 

Fig. 4. Discriminatory power of the various methods of estimating erythrocytic galactose-l-phosphate uridyl transferase. Methods (a) Anderson et al. (A2) (b) Bretthauer et al. (B17) (c) Kirkman and Bynum (KLO), and Hsia et al. (H12) and (d) Schwarz et al. (S12).

The extent to which the level of galactose-l-phosphate uridyl transferase activity in erythrocytes reflects the level in the liver is unknown except for a bandful of cases. This problem is worth further investigation with particular reference to these 2 groups of heterozygotes, those showing unusual galactose intolerance and those with very low enzymatic activity. 

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