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Stability of some Pyridoxal Phosphate-dependent Enzymes in Vitamin B-6 Deficient Rats¹

Departments of Biochemistry and Nutritional Sciences, University of Wisconsin-Madison, Madison, Wisconsin 53706

The effects of lowering the liver pyridoxal phosphate (PLP) concentration by vitamin B-6 deficiency on the stability of several rat liver enzymes were examined. Three PLP-dependent enzymes (serine dehydratase, ornithine--aminotransferase, and tyrosine aminotransferase) and two non-PLP-dependent enzymes (glucose-6-phosphate dehydrogenase and phosphoenolypyruvate carboxykinase) were induced in vitamin B-6 deficient and control rats by feeding them high-protein diets or by injecting them with glucagon or dexamethasone. The decline of each activity was followed after withdrawal of the inducer. Serine dehydratase activity declined more rapidly in vitamin B-6 deficient than in control liver; however, ornithine aminotransferase and tyrosine aminotransferase activities were equally stable in deficient and control liver. Ornithine aminotransferase was predominantly in holoenzyme form in both control and deficient rats, whereas tyrosine aminotransferase was predominantly in apoenzyme form in both groups. The proportion of scrine dehydratase in apoenzyme form was twice as great in deficient as in control liver. The apoenzyme was less stable than the holoenzyme. Activity changes of glucose-6-phosphate dehydrogenase and phosphoenolpyruvate carboxykinase in control and vitamin B-6 deficient rats were similar. The results suggest that differences in the stability of PLP-dependent enzymes in vitamin B-6 deficient rats depend upon differences in the proportions of these enzymes existing as holo- and apoenzyme.

KEY WORDS: • vitamin B-6 deficiency • enzyme stability • protein degradation • pyridoxal phosphate

¹ Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, and by U.S. Public Health Service Grant AM 10748 from the National Institute of Arthritis, Metabolism and Digestive Diseases.

² Present address: The Procter and Gamble Company, Miaml Valley Laboratories, P.O. Box 39175. Cincinnati, Ohio 45239.

Manuscript received 7 August 1975.