Protection of tyrosine aminotransferase against proteolytic digestion by nucleotide derivatives

The abilities of several nucleotides to protect tyrosine aminotransferase (L-tyrosine: 2-oxoglutarate aminotransferase, EC 2.6.1.5) against proteolytic inactivation in vitro have been examined as part of an ongoing investigation of the role of cyclic GMP in the intracellular degradation of the hepatic enzyme. Although neither cyclic GMP nor cyclic AMP was found to exert such a protective effect, certain nucleotide analogs were observed to inhibit the inactivation of tyrosine aminotransferase by trypsin and chymotrypsin. The nucleotides which conferred the strongest protection were the dibutyryl derivatives of cyclic GMP and cyclic AMP. This phenomenon appears to require a purine nucleotide with hydrophobic substituent(s), while the cyclic phosphate is not essential. The nucleotides probably act by direct interaction with tyrosine aminotransferase as indicated by changes in kinetic properties and heat stability of the enzyme and by their failure to inhibit trypsin when other protein substrates, including another aminotransferase, were used. Dibutyryl cyclic AMP was shown to block the appearance of a characteristic 43 kDa tryptic cleavage product of tyrosine aminotransferase but not the conversion of the native 54 kDa form to a size of 50 kDa. Arguments are presented against the involvement of the protective effect in the actions of dibutyryl cyclic nucleotides on tyrosine aminotransferase in cells.