Purine-Metabolizing Enzymes of Tetrahymena Pyriformis

Except for the recent report of Friedkin on pyrimidine nucleoside phosphorylase (2) and several papers from this laboratory dealing with ribonuclease and deoxyribonuclease (3, 4), little information is available concerning the occurrence of nucleolytic and nucleosynthetic enzymes in the ciliated protozoan, Tetrahymena pyrif0rmis.lThis is in contrast to a number of studies carried out on the purine and pyrimidine growth requirements of the organism in synthetic media with labeled and non-labeled substrates.Thus, it has been shown that Tetrahymena requires an exogenous source of guanine (5) (guanosine, guanylic acid), and that xanthine (5) and xanthosine ( 6) are nutritionally inert.While such growth experiments constitute an important approach to the metabolism of nucleic acid intermediates, strong supporting evidence for the occurrence of various purine interconversions may be obtained by the demonstration of specific enzymes in extracts of the organisms.The purpose of the present study is to survey cell-free preparations obtained from T. pyriformis for the presence of several purine-metabolizing enzymes known to occur in vertebrates, invertebrates, and microorganisms, and to determine some of the properties of these enzymes. MethodsCultures and Preparation of Homogenates-These procedures were performed exactly as described previously (7).Enzyme assays were carried out essentially according to the spectrophotometric principles outlined by Kalckar (8-10).For convenience, the components of each test syst,em n-ill be presented separately for each enzyme in the s&ion, 'LResults."React,ions were followed in a model DU Becknlan spcc't,rophot,ometer f-it&d \ a block in which water circulated f'~om a chonstant tempcrat;ure bath maintained at 24.8" f 0.2".All reactants except enzyme were incubated in the water bath prior to addition