C-esterases

In thrombotic episodes accompanied by activation of complement, C, -esterase inhibitor is consumed by binding to the C,-esterase that is formed, leading to an increase in the C(-esterase inhibitor complex (93). 

Mammalian esterases have been classified into three groups according to specificity for substates and inhibitors (110). In terms of overall hydrolytic activity in mammals, the most important class of esterases is that of the B-esterases, which are principally active with aliphatic esters and amides. A-Esterases are important for aromatic esters and organophosphorus esters, and C-esterases are active with acetyl esters. In general, the specificity of mammalian esterases is determined by the nature of substituent groups (acetyl, alkyl, or aryl) rather than the heteroatom (O, N, or S) that is adjacent to the carboxy group. That is, the same esterase would likely catalyze hydrolysis of an ester, amide, or thioester as long as the substituents were identical except for the heteroatom (110). 

Acetylesterase Acetic-ester acetylhydrolase, C-esterase Acetic acid esters... 

A classification based on the interaction of esterases with organophosphates (2.1) has been introduced by Aldridge [64] class A-esterases hydrolyze organophosphate esters while B-esterases are irreversibly inhibited by them. Another, lesser-used criterion, is the effect of sodium 4-(hydroxymer-curio)benzoate or Hg2+, which inhibits A-esterases but has little effect on B-esterases. Class C-esterases, enzymes that do not interact at all with organophosphates, have been added to the classification system [64][65],... 

C-Esterases are found in the IUBMB classification under the entry acetyl-esterase (acetic ester acetylhydrolase, EC 3.1.1.6). Doubts have been expressed about the existence of C-esterases [56], Indeed, their activation by sodium 4-(hydroxymercurio)benzoate is not reproducible, and paraoxon has at least slight inhibitory effects on these esterases. 

In normal milk, the ratio of A B C esterase activity is about 3 10 1 but the level of A-esterase activity increases considerably on mastitic infection. A and C esterases are considered to be of little technological significance in milk. 

Various esterases exist in mammalian tissues, hydrolyzing different types of esters. They have been classified as type A, B, or C on the basis of activity toward phosphate triesters. A-esterases, which include arylesterases, are not inhibited by phosphotriesters and will metabolize them by hydrolysis. Paraoxonase is a type A esterase (an organophosphatase). B-esterases are inhibited by paraoxon and have a serine group in the active site (see chap. 7). Within this group are carboxylesterases, cholinesterases, and arylamidases. C-esterases are also not inhibited by paraoxon, and the preferred substrates are acetyl esters, hence these are acetylesterases. Carboxythioesters are also hydrolyzed by esterases. Other enzymes such as trypsin and chymotrypsin may also hydrolyze certain carboxyl esters. 

In view of the large number of esterases in many tissues and subcellular fractions, as well as the large number of substrates hydrolyzed by them, it is difficult to derive a meaningful classification scheme. The division into A-, B-, and C- esterases on the basis of their behavior toward such phosphate triesters as paraoxon, first devized by Aldridge, is still of some value, although not entirely satisfactory. 

Although several reports concerning esterases in milk have appeared in the literature, little detailed information on the individual enzymes is available. Arylesterase or A-esterase (EC 3.1.1.2), carboxylesterase or B-esterase (EC 3.1.1.1), and cholinesterase or C-esterase (EC 3.1.1.7 EC 3.1.1.8) have been identified (Forster et al., 1961 Kitchen, 1971 Nakanishi and Tagata, 1972 Deeth, 1978). 

Esterases that metabolize organophosphates can be divided into three groups A-esterases, which are not inhibited by organophosphates but hydrolyze them B-esterases, which are susceptible to organophosphate inhibition and C-esterases, which are uninhibited by organophosphates and do not degrade them. 

Regarding the more definitive weapon of the complement cascade, it has recently been demonstrated that the administration of supplemental sodium ascorbate to guinea pigs significantly increases the esteratic power of the first component of complement (C, esterase activity, without which the whole complement cascade is inoperable) in animals immunized with antipenicilloyl (guinea pig)-y-globulin. ... 

Muchmore E, Varki A (1987) Selective inactivation of influenza C esterase a prohe for detecting 9-0-acetylated sialic acids. Science 236 1293-1295... 

Vlasak, R., Muster, T, Lauro, A. M., Powers, J. C., and Palese, P., 1989, Influenza C esterase Analysis of catalytic site, inhibition and possible function, J. Virol. 63 2056-2062. 

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