The oxidation in liver of l-tyrosine to acetoacetate through p-hydroxyphenylpyruvate and homogentisic acid.

The picture of L-tyrosine metabolism obtained from isotope experiments and other studies on intact animals is still incomplete. Either p-hydroxyphenylpyruvate or 2:5-dihydroxyphenylalanine may be the first intermediate, since either could form the second postulated intermediate, 2:5-dihydroxyphenylpyruvate (Neubauer, 1909), and both are converted to homogentisic acid by alcaptonurics (Neuberger, Rimington & Wilson, 1947). p-Hydroxyphenylpyruvate is also known to be formed from tyrosine in scorbutic guinea pigs and premature infants (Sealock & Silberstein, 1940; Levine, Marples & Gordon, 1941) and was excreted by Medes's case of tyrosinosis (Medes, 1932). However, the evidence for the assumption that normal tyrosine metabolism goes through these homogentisic acid precursors and homogentisic acid itself is only indirect. Normal liver can oxidize homogentisic acid to acetoacetate (Edson, 1935b; Zom, 1940; Ravdin & Crandall, 1950). Still more significant is the isotopic labelling of acetoacetate formed from labelled phenylalanine by liver slices (Schepartz & Gurin, 1949; Weinhouse & Millington, 1948), which shows that a molecular rearrangement ofthe side chain must have occurred. This rearrangement is similar to that which must be postulated for the intermediate formation of homogentisic acid from tyrosine (Meyer, 1901; Friedman, 1908). Investigations of the oxidation of L-tyrosine in liver homogenates have not resolved any of the uncertainties about the reaction. There is agreement that the oxidation ofL-tyrosine by liver homogenates uses four atoms ofoxygen and produces one molecule each of carbon dioxide and acetoacetate but no ammonia (Bernheim & Bernheim, 1934; Felix, Zorn & Dirr-Kaltenbach, 1937; Zorn, 1940; Felix & Zorn, 1941; Sealock & Goodland, 1949). In liver slices fumaric acid (or malic acid) and acetoacetate are formed from L-tyrosine (Lerner, 1949). Felix & Zorn (1941) also found alanine to be a product of the reaction, and they thought that this arose from a splitting-off of the tyrosine side chain. Isotope experiments have failed to confirm this origin of the alanine (Lerner, 1949). The most thorough investigations of the intermediate steps (Felix et at. 1937; Zorn, 1940) have apparently excluded both p-hydroxyphenylpyruvate and homogentisic acid as intermediates. Their investigations suggest a totally different pathway of L-tyrosine oxidation in liver than that deduced from intact animal experiments. A more active L-tyrosine oxidation system than any hitherto used, freed from other reactions occurring in crude liver homogenates, was needed to investigate these discrepancies. Concentrated suspensions of liver homogenates have always been used to obtain the highest tyrosine oxidation activity (Bernheim & Bernheim, 1934; Sealock & Goodland, 1949), presumably to avoid dilution of unidentified dialysable components of the system. Using such suspensions, Rienits (1950) and Painter & Zilva (1950) have shown that addition of ascorbic acid gave a small but statistically significant increased oxidation of L-tyrosine in liver preparations from both scorbutic and normal guinea pigs. The recent identification of a-ketoglutarate and ascorbic acid as the major co-factors for L-tyrosine oxidation in liver homogenates (LeMay-Knox & Knox, 1951) has permitted a higher order of activity to be obtained. These additions to the reaction have also made possible some purification of the system and almost complete elimination of secondary reactions. With such preparations the pathway of tyrosine oxidation in liver has been re-investigated.