SALVAGE PATHWAY OF PYRIMIDINE SYNTHESIS - DIVERGENCE OF SUBSTRATE-SPECIFICITY IN 2 RELATED SPECIES OF TELEOSTEAN FISH

For nucleotide synthesis, cells use purine and pyrimidine nucleosides generated either through de novo synthesis or through utilization of s alvage pathways. In the pyrimidine salvage pathway, thymidine is taken up by transport proteins and phosphorylated by the enzyme thymidine k inase to thymidine monophosphate. So far, all vertebrates analyzed are able to use radioactively labeled thymidine for the biosynthesis of n ucleotides in brain tissue. However, when standard autoradiographic, i mmunohistochemical and biochemical procedures were applied for the det ection of the incorporation of tritiated thymidine and the thymidine a nalogue 5-bromo-2'-deoxyuridine into DNA to two species of gymnotiform fish, a divergence in substrate specificity has been revealed. Althou gh brain cells of the two species, Apteronotus leptorhynchus and Eigen mannia sp., can utilize 5-bromo-2'-deoxyuridine for pyrimidine synthes is, only Eigenmannia sp. is able to incorporate tritiated thymidine in to DNA during the S phase of the cell cycle. We hypothesize that this inability to use thymidine for nucleotide synthesis is caused either b y a defect in the transport system mediating the uptake of thymidine o r by a deficiency in the thymidine kinase of A. leptorhynchus.