The paradoxical finding of tyrosinemia and increased urinary p-hydroxyphenylpyruvate in a patient whose liver biopsy contained normal p-hydroxyphenylpyruvate hydroxylase and virtual absence of soluble tyrosine aminotransferase led to studies of the latter enzyme.Two new methods for the assay of tyrosine aminotransferase are described together with the results of studies of tissue and subcellular distribution of this enzyme in human and rat tissues. Only a mitochondrial form of tyrosine aminotransferase was present in the patient's liver. Both soluble and mitochondrial forms et al. ( 1958) described the occurrence of tyrosine aminotransferase* 1•2 in the supernatant of rat liver homogenates after centrifugation at 13,000-14,OOOg. This form of the mammalian enzyme has been studied extensively and has been shown to be under diurnal (Wurtman and Axelrod, 1967) and hormonal control (Lin and
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