UDP hydrolase activity associated with the porcine liver annexin fraction

In the crude fraction of porcine liver annexins, we identified annexin IV (AnxIV), AnxII and AnxVI of MW (molecular weight) of 32, 36 and 68 kDa, respectively, an albumin of MW of 61.5 kDa and an UDP hydrolase (UDPase) of MW of 62 kDa, related to the human UDPase from Golgi membranes. The latter enzyme exhibits its highest specificity towards UDP and GDP but not ADP and CDP, and it is stimulated by Mg(2+) and Ca(2+). AnxVI itself, although it binds purine nucleotides, does not exhibit hydrolytic activity towards nucleotides. Taken together, these results suggest that AnxVI may interact in vivo with a nucleotide-utilizing enzyme, UDPase. This is in line with observations made by other investigators that various annexins are able to interact with nucleotide-utilizing proteins, such as protein kinases, GTPases, cytoskeletal proteins and p120(GAP). Such interactions could be of particular importance in modulating the biological activities of these proteins in vivo.