Tyrosine aminotransferase converting factor: kinetic properties, cellular localization, and tissue distribution.

Utilizing a more rapid procedure for determining tyrosine aminotransferase-converting factor activity, the kinetic properties of this factor were characterized further. Tyrosine aminotransferase-converting factor is a heat-labile substance present in the particulate fraction of rat liver that converts tyrosine aminotransferase form III to I at 4 degrees C. Analysis of the distribution of marker enzymes for mitochondria and lysosomes, and of converting factor, following differential and discontinuous sucrose gradient centrifugation indicated that this factor was associated with light lysosomes. The activity of converting factor was not altered following administration of cortisol. Converting factor activity, equivalent to that in liver, was also observed in particulate fractions from kidney and spleen, and to a lesser extent, in pancreas and salivary gland. No detectable activity was observed in brain, heart, small intestine, skeletal muscle, red blood cells, serum, or plasma. The presence of converting factor activity in kidney and spleen suggests that other proteins are substrates for this factor since tyrosine aminotransferase is virtually absent from these tissues. Alternatively, the absence of converting factor from other tissues need not indicate they are devoid of converting factor-like activity merely, that such activity in these tissues has different specificities and does not utilize tyrosine aminotransferase as a substrate.