Hypotaurine conversion

Further investigations with boar seminal fluid performed by Kalic et al. [54] using a 600-MHz NMR instrument revealed the identification of the amino acids hypotaurine and carnitine. Semiselective HSQC 2D experiments on an 800-MHz speedometer [55] enable one to differentiate between the structurally similar polyamines, spermidine, spermine and their metabolic precursor putrescine, all attracting interest for tumor therapy, in seminal fluid. Recently, Tomlins et al. [56] studied dynamic biochemical processes in incubated human seminal fluid samples The enzymatic hydrolysis of phosphoiylcholine to choline and the conversion of uridine-5 -monophosphate to uridine were found to be very fast, whereas the slow polypeptide hydrolysis to amino acids can be inhibited by the addition of EDTA. It remains to be seen which role the biochemical changes play in the reproductive function. 

Fig. 20.3 Pathway of methionine metabolism. The numbers represent the following enzymes or sequences (1) methionine adenosyltransferase (2) S-adenosylmethionine-dependent transmethylation reactions (3) glycine methyltransferase (4) S-adenosylhomocysteine hydrolase (5) betaine-homocysteine methyltransferase (6) 5-methyltetrahydrofolate homocysteine methyltransferase (7) serine hydroxymethyltransferase (8) 5,10-methylenetetrahydrofolate reductase (9) S-adenosylmethionine decarboxylase (10) spermidine and spermine synthases (11) methylthio-adenosine phosphorylase (12) conversion of methylthioribose to methionine (13) cystathionine P-synthase (14) cystathionine y-lyase (15) cysteine dioxygenase (16) cysteine suplhinate decarboxylase (17) hypotaurine NAD oxidoreductase (18) cysteine sulphintite a-oxoglutarate aminotransferase (19) sulfine oxidase. MeCbl = methylcobalamin PLP = pyridoxal phosphate...

In order to characterize more closely the oxidation reaction of hypotaurine, we incubated homogenates of different tissues of adult mice with [ S]hypotaurine. These first incubations were carried out at 310 K and pH 7.4 for up to 4 h in open tubes without cofactors. We were unable for some time to convince ourselves whether or not there occurs any conversion of hypotaurine to taurine under such conditions. We continued tenaciously with our experiments, however, because hypotaurine was so rapidly converted to taurine in vivo. Only after numerous trials could we infer that there occurred some conversion also in vitro, but its detection was just at the sensitivity limits of our assay. It is thus no wonder that Cavallini et al. (1954) had earlier failed to detect any. Only 2-3% of the added hypotaurine were oxidized by homogenates of the liver and kidney, about 1% by the heart, spleen, muscle and lung, and still less by the brain. These traces of activity disappeared when the tissue samples were boiled or incubated at 273 K. The activity was slightly enhanced by NAD , NADP" " or oxidized glutathione, whereas reduced glutathione, NADH and NADPH had no effect. 

In the present studies we have shown that the conversion of hypotaurine to taurine in mouse tissues, particularly in liver and brain, exhibits characteristic properties of an enzyme-catalyzed reaction. It is pH and temperature-dependent, obeys Michaelis-Menten kinetics and is enhanced by coenzymes, metal activators and... 

This pathway awaits experimental validation (Huxtable 1978). Furthermore, as discussed above, no enzymatic activity has been found for the conversion of hypotaurine to taurine. Although hypotaurine spontaneously converts to taurine, it appears unlikely that an important step in biosynthesis would be nonenzymatic. 

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