The tyrosine oxidation system of liver. I. Extracts of rat liver acetone powder.

Many investigators of tyrosine oxidation have used systems in vitro such as slices, minces, and, more recently, homogenates of liver. During our search for a suitable preparation other than liver slices for studying the conversion of phenylalanine to tyrosine, an extract of rat liver acetone powder was tested. Although the phenylalanine to tyrosine conversion was not demonstrated, it was found that the oxidation of tyrosine was catalyzed by the powder extract. This paper will report on the conditions which were found necessary to study tyrosine oxidation by this system. Very few papers in the literature deal with the use of acetone powder extracts as a system for studying tyrosine oxidation. Lang and Westphal (1) reported that an enzyme which attacked n-phenylalanine and L-tyrosine was present in some acetone powder preparations of dog liver. Felix et al. (2) used an acetone powder of pig kidney in their study of tyrosine oxidation. Although Felix and Schaefer reported later (3) that the supernatant of centrifuged minced rat liver had very little action on either n-tyrosine or p-hydroxyphenylpyruvic acid, a fact which suggested that some of the enzymes involved are associated with the insoluble liver fraction, more recent work with homogenates (4, 5) indicates that the enzymes involved in tyrosine oxidation are soluble. Our results support this view. For the study of tyrosine oxidation, the acetone powder extract has several advantages over homogenates. The powder is very stable for at least several weeks even at room temperature, and the extract contains the enzymes in a concentrated solution and therefore the reactions can be studied in the ordinary sized Warburg vessels with up to 10 pM of tyrosine. The acetone treatment and extraction steps remove or inactivate many of the other enzyme systems. As a result, the control flasks have a much lower oxygen uptake and the system is more specific for the oxidation of tyrosine. Lastly, the extract will serve as a convenient starting point for fractionation