Effect of dimethylaminoethylphosphonate on phospholipid metabolism in Tetrahymena

Dimethylaminoethylphosphonate (DMAEP) was incorporated into the phospholipids of the ciliate protozoan Tetrahymena thermophila at the expense of both phosphatidylethanolamine and phosphatidylcholine, but it had no effect on the levels of the 2-aminoethylphosphonolipid. The newly formed DMAEP-lipid accounted for almost 50% of the phospholipids of the organism. The DMAEP was incorporated into the phospholipids using both the ethanolaminephosphotransferase and cholinephosphotransferase pathways. The DMAEP-lipid was not methylated to the trimethyl derivative, confirming the lack of methylation of phosphonolipids by Tetrahymena.     

Effect of tolbutamide on the activity of transglutaminase

Five obese patients were studied during 7 days, 750 mg of tolbutamide, per os, was given. Blood samples were drawn at basal state and at 3, 5, 7 days during the treatment and 6 days after it. The values of transglutaminase activity (that in the basal state were similar to that in the controls) decreased significantly at the seventh day of treatment (72.3%). This decrease was transient and rapidly returned to the basal values when the drug was suspended. The results suggest that sulfonylureas exert in part their hypoglucemic effect by modificating the insulin receptor binding through the inhibition of transglutaminase activity.     

Effect of a phosphonic acid analog of choline phosphate on phospholipid metabolism in Tetrahymena

When Tetrahymena thermophila is grown on a medium containing increasing concentrations of N,N,N-trimethyl-2-aminoethylphosphonate (TMAEP), up to 60% of the choline phosphate in phosphatidylcholine is replaced by the phosphonic acid. There is an increase in the relative amount of quaternary ammonium-containing lipid (phosphatidylcholine plus TMAEP-lipid) at the expense of phosphatidylethanolamine. There is no effect of the TMAEP on either 2-aminoethylphosphonolipid levels or on de novo 2-aminoethylphosphonate synthesis. Higher levels of TMAEP in the medium (25 and 50 mm) lead to decreased growth of Tetrahymena and to an abnormal cell morphology.     

The metabolism of pyrimidine compounds by Tetrahymena pyriformis

The pyrimidine requirements for growth of T. pyriformis and for reversal of the growth inhibition caused by folate deprivation have been studied. The effects of thymidine and 5-fluorodeoxyuridine have been shown to be quantitatively different from the effects of these compounds on growth and the rate of DNA synthesis in mammalian cells. Labelled nucleosides added to the medium have been found to be converted to the corresponding bases with the exception of deoxycytidine, which is first deaminated to deoxyuridine. As a result no deoxynucleosides other than thymidine specifically label DNA. The results allow deductions to be made concerning the enzymes involved in pyrimidine utilization by this organism. It is suggested that pyrimidine utilization is always channeled through uracil in the case of those compounds that can supply the pyrimidine requirement for growth.     

The purine and pyrimidine metabolism of Tetrahymena pyriformis

The metabolism of purines and pyrimidines by the ciliated protozoan Tetrahymena was investigated with the use of enzymatic assays and radioactive tracers. A survey of enzymes involved in purine metabolism revealed that the activities of inosine and guanosine phosphorylase (purine nucleoside: orthophosphate ribosyltransferase, E.C. 2.4.2.1) were high, but adenosine phosphorylase activity could not be demonstrated. The apparent K_m for guanosine in the system catalyzing its phosphorolysis was 4.1 ± 0.6 × 10^?3 M. Pyrophosphorylase activities for IMP and GMP (GMP: pyrophosphate phosphoribosyltransferase, E.C. 2.4.2.8), AMP (AMP: pyrophosphate phosphoribosyltransferase, E.C. 2.4.2.7), and 6-mercaptopurine ribonucleotide were also found in this organism; but a number of purine and pyrimidine analogs did not function as substrates for these enzymes. The metabolism of labeled guanine and hypoxanthine by intact cells was consistent with the presence of the phosphorylases and pyrophosphorylases of purine metabolism found by enzymatic studies. Assays for adenosine kinase (ATP: adenosine 5'-phosphotransferase, E.C. 2.7.1.20) inosine kinase, guanosine kinase, xanthine oxidase (xanthine: O₂ oxidoreductase, E.C. 1.2.3.2), and GMP reductase (reduced-NADP: GMP oxidoreductase [deaminating], E.C. 1.6.6.8) were all negative. In pyrimidine metabolism, cytidine-deoxycytidine deaminase (cytidine aminohydrolase, E.C. 3.5.4.5), thymidine phosphorylase (thymidine: orthophosphate ribosyltransferase, E.C. 2.4.2.4), and uridine-deoxyuridine phosphorylase (uridine: orthophosphate ribosyltransferase, E.C. 2.4.2.3) were active; but cytidine kinase, uridine kinase (ATP: uridine 5'-phosphotransferase, E.C. 2.7.1.48), and CMP pyrophosphorylase could not be demonstrated.