Endogenous factor in rat liver that converts tyrosine aminotransferase form III to form I: a rapid assay of this converting factor

Effect of physiological reducing compounds on the inactivation of tyrosine aminotransferase from guinea-pig liver

1. Tyrosine aminotransferase from guinea-pig liver is inactivated at neutral pH by a factor localized in the microsomal fraction. The inactivation, independent of exogenous L-cysteine, is rapidly reversed by addition of dithiothreitol. 2. The effects of physiological reducing agents on the enzyme inactivation were investigated. L-Cysteine and L-cysteamine enhance the inactivation rate of the enzyme in the presence of microsomal membranes, and also they are able to bring about the loss in enzyme activity independently of microsomal action. Reduced glutathione, at physiological concentration, and NADPH decrease the inactivation rate. Other physiological reducing compounds, as well as oxidized glutathione and NADP+, are without effect. 3. Neither reduced glutathione nor NADPH, unlike dithiothreitol and mercaptoethanol, is able to restore the activity of partially inactivated tyrosine aminotransferase. 4. It is proposed that the intracellular concentration of reduced glutathione might modulate the rate of inactivation of the enzyme in vivo.

Purification and characterization of a new thiol proteinase from rat kidney

The levels of activity of a new proteinase, termed cathepsin T, in rat tissues were examined. This enzyme had been previously reported to exist in rat liver on the basis of its catalysis of the conversion of multiple forms of tyrosine aminotransferase k(L-tyrosine: 2-oxoglutarate aminotransferase, EC 2.6.1.5). Kidney was found to be the richest source of cathepsin T activity, exhibiting cathepsin T activity in tissues was kidney much greater than spleen greater than liver greater than small intestine greater than lung. The proteinase activity was not detectable in heart, skeletal muscle, brain and blood. Kidney cathepsin T was purified about 1400-fold to homogeneity with a 20% yield by the purification procedure similar to that used for the liver enzyme (Gohda, E. and Pitot, H.C. (1980) J. Biol. Chem. 255, 7371-7379). Cathepsin T purified from rat kidney was found to be a glycoprotein, the molecular weight of which was between 33,500 and 35,000. Purified kidney cathepsin T converted Form I of tyrosine aminotransferase in the same way as the liver proteinase, with concomitant conversion of 52,500 dalton subunits of the aminotransferase to 48,000 dalton subunits. Kidney cathepsin T showed the same specific activities toward Form I of tyrosine aminotransferase, casein and acid-denatured hemoglobin as did the liver form of the enzyme. Many other characteristics common to the proteinases purified both from rat kidney and liver were found. We have concluded that kidney cathepsin T is the same enzyme as the liver proteinase.