Different substrate specificity of two isozymes of cytosolic 5'-nucleotidase from rabbit heart.

Two isozymes of soluble cytoplasmic 5’-nucleotidase (5’-NT) have been found in the heart of several species1, 2, including man (Skladanowski, submitted). AMP- and IMP-specific fonns (also coded N-I and N-II), having KM’ s at milimolar range, are thought to be involved in the control of intracellular levels of AMP and IMP3. AMP-specific isozyme has been thoroughly characterized after purification from pigeon4, rabbit5 and dog6 hearts. Physiological roles plausibly played in the heart by two distinct isoforms could be: production of ‘retaliatory’ adenosine7 by AMP-specific 5’-NT, and controlling of the overall purine degradation rate by IMP-specific 5’-NT (for the review on 5-NT see ref.8).