Evidence for the extreme overestimation of choline acetyltransferase in human sperm, human seminal plasma and rat heart: a case of mistaking carnitine acetyltransferase for choline acetyltransferase.
Detection of choline acetyltransferase (ChAc) in a number of non-neuronal tissues has been extremely overestimated. There are two major types of errors encountered. Type 1 error occurs when endogenous substrates (e.g. L-carnitine) are acetylated by acetyltransferase enzymes (e.g. carnitine acetyltransferase ( CarAc ) ) yielding an acetylated product mistaken for acetylcholine (AcCh). In the past, human sperm and human seminal plasma putative ChAc activity has been extremely overestimated due to Type 1 error. This study demonstrates (1) an endogenous acetyltransferase and substrate activity in human sperm and human seminal plasma forming an acetylated product that is not AcCh but probably acetylcarnitine ( AcCar ); (2) that the addition of 5 mM choline substrate does not significantly increase acetyltransferase activity; (3) that boiled seminal plasma contains an endogenous acetyltransferase substrate which is not choline, but probably L-carnitine. Type 2 error occurs when endogenous carnitine acetyltransferase synthesizes true AcCh, resulting in mistaken evidence for ChAc. This is demonstrated by the fact that the choline substrate Km-value for the neuronal or true ChAc from mouse brain is 0.73 +/- 0.06 mM while the Km-value of choline substrate for purified CarAc from pigeon breast muscle is 108 +/- 4 mM. Type 2 error has occurred for the estimation of putative ChAc in rat heart. The rat heart ChAc was measured in previous studies utilizing a concentration of 30 mM choline substrate. While saturation of neuronal ChAc is observed at 2-5 mM choline, saturation of the rat heart CarAc enzyme is not reached until over 800 mM. Purified CarAc significantly synthesizes AcCh at 30 mM choline. Thus, putative ChAc has been greatly overestimated in the scientific literature for mammalian sperm, human seminal plasma and rat heart.
D000107:Acetylation; D000123:Acetyltransferases / Q000032:analysis*; D000328:Adult; D000818:Animals; D001921:Brain / Q000201:enzymology; D002331:Carnitine / Q000378:metabolism; D002332:Carnitine O-Acetyltransferase / Q000032:analysis*; D002794:Choline / Q000378:metabolism; D002795:Choline O-Acetyltransferase / Q000032:analysis*; D005189:False Positive Reactions; D006801:Humans; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D009206:Myocardium / Q000201:enzymology*; D051381:Rats; D011919:Rats, Inbred Strains; D012661:Semen / Q000201:enzymology*; D013094:Spermatozoa / Q000201:enzymology*; D013379:Substrate Specificity
D000123:Acetyltransferases; D002795:Choline O-Acetyltransferase; D002332:Carnitine O-Acetyltransferase; D002794:Choline; D002331:Carnitine