Discrimination of liver cirrhosis from chronic hepatitis by analysis of serum cholinesterase isozymes using affinity electrophoresis with concanavalin A or wheat germ agglutinin

Blood cholinesterases as human biomarkers of organophosphorus pesticide exposure

The organophosphorus pesticides of this review were discovered in 1936 during the search for a replacement for nicotine for cockroach control. The basic biochemical characteristics of RBC AChE and BChE were determined in the 1940s. The mechanism of inhibition of both enzymes and other serine esterases was known in the 1940s and, in general, defined in the 1950s. In 1949, the death of a parathion mixer-loader dictated blood enzyme monitoring to prevent acute illness from organophosphorus pesticide intoxication. However, many of the chemical and biochemical steps for serine enzyme inhibition by OP compounds remain unknown today. The possible mechanisms of this inhibition are presented kinetically beginning with simple (by comparison) Michaelis-Menten substrate enzyme interaction kinetics. As complicated as the inhibition kinetics appear here, PBPK model kinetics will be more complex. The determination of inter- and intraindividual variation in RBC ChE and BChE was recognized early as critical knowledge for a blood esterase monitoring program. Because of the relatively constant production of RBCs, variation in RBC AChE was determined by about 1970. The source of plasma (or serum) BChE was shown to be the liver in the 1960s with the change in BChE phenotype to the donor in liver transplant patients. BChE activity was more variable than RBC AChE, and only in the 1990s have BChE individual variation questions been answered. We have reviewed the chemistry, metabolism, and toxicity of organophosphorus insecticides along with their inhibitory action toward tissue acetyl- and butyrylcholinesterases. On the basis of the review, a monitoring program for individuals mixing-loading and applying OP pesticides for commercial applicators was recommended. Approximately 41 OPs are currently registered for use by USEPA in the United States. Under agricultural working conditions, OPs primarily are absorbed through the skin. Liver P-450 isozymes catalyze the desulfurization of phosphorothioates and phosphorodithioates (e.g., parathion and azinphosmethyl, respectively) to the more toxic oxons (P = O(S to O)). In some cases, P-450 isozymes catalyze the oxidative cleavage of P-O-aryl bonds (e.g., parathion, methyl parathion, fenitrothion, and diazinon) to form inactive water-soluble alkyl phosphates and aryl leaving groups that are readily conjugated with glucuronic or sulfuric acids and excreted. In addition to the P-450 isozymes, mammalian tissues contain ('A' and 'B') esterases capable of reacting with OPs to produce hydrolysis products or phosphorylated enzymes. 'A'-esterases hydrolyze OPs (i.e., oxons), while 'B'-esterases with serine at the active center are inhibited by OPs. OPs possessing carboxylesters, such as malathion and isofenphos, are hydrolyzed by the direct action of 'B'-esterases (i.e., carboxylesterase, CaE). Metabolic pathways shown for isofenphos, parathion, and malathion define the order in which these reactions occur, while Michaelis-Menten kinetics define reaction parameters (Vmax, K(m)) for the enzymes and substrates involved, and rates of inhibition of 'B'-esterases (kis, bimolecular rate constants) by OPs and their oxons. OPs exert their insecticidal action by their ability to inhibit AChE at the cholinergic synapse, resulting in the accumulation of acetylcholine. The extent to which AChE or other 'B'-esterases are inhibited in workers is dependent upon the rate the OP pesticide is activated (i.e., oxon formation), metabolized to nontoxic products by tissue enzymes, its affinity for AChE and other 'B'-esterases, and esterase concentrations in tissues. Rapid recovery of OP BChE inhibition may be related to reactivation of inhibited forms. AChE, BChE, and CaE appear to function in vivo as scavengers, protecting workers against the inhibition of AChE at synapses. Species sensitivity to OPs varies widely and results in part from binding affinities (Ka) and rates of phosphorylation (kp) rather than rates of activation and detoxif

A missense mutation in the FUT6 gene results in total absence of alpha3-fucosylation of human alpha1-acid glycoprotein

The major alpha3-fucosyltransferase activity in human plasma is encoded by the gene for fucosyltransferase VI (FUT6). A missense mutation (Gly-739 --> Ala) in this gene is responsible for deficiency of enzyme activity in plasma. To examine whether this fucosyltransferase is the sole enzyme responsible for the alpha3-fucosylation of serum glycoproteins in the liver, we studied the fucosylation of three glycoproteins in sera of individuals with or without inactivated FUT3 and/or FUT6 gene(s) but with a functional FUT5 gene. alpha1-Acid glycoprotein was used as the principal reporter protein for liver alpha3-fucosyltransferase activity, because of its high fucose content. In all individuals with the FUT6 missense mutation Gly-739 --> Ala in double dose, no fucosylation of alpha1-acid glycoprotein was found. This alpha1-acid glycoprotein was not intrinsically resistant to fucosylation, since it was susceptible to in vitro fucosylation using an alpha3/4-fucosyltransferase isolated from human milk. The same result was found for alpha1-antichymotrypsin and alpha1-protease inhibitor. On the other hand in all individuals with alpha3-fucosyltransferase activity in plasma, alpha3-fucosylated glycoforms of the glycoproteins studied were found. The degree of fucosylation of alpha1-acid glycoprotein was correlated with alpha3-fucosyltransferase activity (Rs = 0.82). These data indicate that the product of FUT6, but not of FUT3 or of FUT5, is responsible for the alpha3-fucosylation of glycoproteins produced in liver and suggest that this organ is a major source of alpha3-fucosyltransferase activity in plasma.

Increased plasma alpha (1 --> 3)-L-fucosyltransferase activities in patients with hepatocellular carcinoma

Alpha (1 --> 3)-L-fucosyltransferase (alpha 1,3FT) activity was determined in plasma of patients with chronic liver diseases, namely, chronic hepatitis (CH), liver cirrhosis (LC) and hepatocellular carcinoma (HCC), The plasma alpha 1,3FT activity was significantly higher (p < 0.01) in chronic liver diseases than that in normal controls. The enzyme activity in plasma of patients with HCC was also significantly higher than that in LC (p < 0.05) or that in CH (p < 0.01). However, no significant difference was observed in the enzyme activity between LC and CH. Plasma alpha 1,3FT activity in patients with HCC was not significantly changed before and after transcatheter arterial embolization. In addition, the enzyme activity in the homogenate of the cirrhotic liver tissue was higher than that in the preparation of the hepatoma tissue in the same patient. These results suggest that the increased plasma alpha 1,3FT activity in patients with HCC reflects mainly the enzyme activity of cirrhotic liver tissue, not that of hepatoma tissue. The significance of the elevated levels of plasma alpha 1,3FT and its decreased hepatoma tissue activity in patients with HCC, compared with that in LC, remains to be clarified.