Purine and Pyrimidine 5′‐Nucleotide Catabolism in Tetrahymena pyriformis W1

Abstract

SYNOPSIS Deamination at pH 7.5 of adenosine, deoxyadenosine, cytidine and deoxycytidine by cell‐free preparations of Tetrahymena pyriformis W was observed both in the presence and absence of fluoride. Deamination of 5′‐AMP, 5′‐dAMP, 5′‐CMP, and 5′‐dCMP was found only in the absence of fluoride. Dephosphorylation of the above nucleotides by acid phosphatases occurred at pH 4.5; reduced activity was noted at pH 7.5. Fluoride effectively blocked acid phosphatase activity at both pH values. This correlation of phosphatase and deaminase activities suggests a catabolic pathway for 5′‐AMP and 5′‐CMP whereby dephosphorylation precedes deamination. Radiolabelled substrates were used to test this hypothesis. The experiments were designed so that conversion of as little at 1.0% of the radiolabelled substrate to the deaminated product could be detected. No 5′‐IMP or 5′‐UMP, the expected deamination products of 5′‐AMP and 5′‐CMP, respectively, was recovered after incubation of the radiolabelled substrates with cell‐free enzyme preparations. Thus, it appears that Tetrahymena has no 5′‐AMP or 5′‐CMP deaminases and that these compounds are deaminated only after conversion to nucleosides. Acid phosphatase activity toward 5′‐GMP, 5′‐dGMP, 5′‐TMP, 5′‐UMP, and 5′‐XMP was also found.

Effects of six pyrimidine analogs on the growth ofTetrahymena thermophilaand their implications in pyrimidine metabolism

“…However, results derived from several early experiments strongly suggest the presence of uridine phosphorylase activity in T. pyriformis [31][32][33]. Whether these discrepancies indicate the presence of a non-canonical uridine phosphorylase in Tetrahymena remains to be further investigated.…”

Effects of six pyrimidine analogs on the growth of Tetrahymena thermophila and their implications in pyrimidine metabolism

“…One clue for this non-interchangeability may be found in the Tetrahymena genome [20] as the key enzyme responsible for interconverting uridine and uracil is absent, according to the KEGG pathway (S3 Fig) [19]. However, results derived from several early experiments strongly suggest the presence of uridine phosphorylase activity in T. pyriformis [33][34][35]. Whether these discrepancies indicate the presence of a non-canonical uridine phosphorylase in Tetrahymena warrants PLOS ONE further investigation.…”