Liver substrate specificities

Uptake of LCFAs across the lipid-bilayer of most mammalian cells occurs through both a passive diffusion of LCFAs and a protein-mediated LCFA uptake mechanism. At physiological LCFA concentrations (7.5 nM) the protein-mediated, saturable, substrate-specific, and hormonally regulated mechanism of fatty acids accounts for the majority (>90%) of fatty acid uptake by tissues with high LCFA metabolism and storage such as skeletal muscle, adipose tissue, liver,... 

Monoamine Oxidases and their Inhibitors. Table 1 Substrate specificity of the two forms of rat liver and brain monoamine oxidase... 

Enzyme preparations from liver or microbial sources were reported to show rather high substrate specificity [76] for the natural phosphorylated acceptor d-(18) but, at much reduced reaction rates, offer a rather broad substrate tolerance for polar, short-chain aldehydes [77-79]. Simple aliphatic or aromatic aldehydes are not converted. Therefore, the aldolase from Escherichia coli has been mutated for improved acceptance of nonphosphorylated and enantiomeric substrates toward facilitated enzymatic syntheses ofboth d- and t-sugars [80,81]. High stereoselectivity of the wild-type enzyme has been utilized in the preparation of compounds (23) / (24) and in a two-step enzymatic synthesis of (22), the N-terminal amino acid portion of nikkomycin antibiotics (Figure 10.12) [82]. 

The hexose phosphate, fructose-1,6-diphosphate, is split by aldolase into two triose phosphates glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. Aldolase consists of four 40-kDa subunits. Three tissue-specific forms exist in human tissues aldolase A (ubiquitous and very active in the muscle), aldolase B (liver, kidney, and small intestine), and aldolase C (specific to the brain). These three isozymes have nearly the same molecular size but differ in substrate specificity,... 

Meier, P. J., et al. Substrate specificity of sinusoidal bile acid and organic anion uptake systems in rat and human liver. Hepatology 1997, 26, 1667-1677. 

Schroeder, A., et al. Substrate specificity of the rat liver Na(+)-bile salt cotransporter in Xenopus laevis oocytes and in CHO cells. Am. J. Physiol. 1998, 274, G370-G375. 

P. J., Hagenbuch, B., Localization of organic anion transporting polypeptide 4 (Oatp4) in rat liver and comparison of its substrate specificity with Oatpl, Oatp2 and Oatp3, Pfliigers Arch. 2001, 443, 188-195. 

Buckholtz, N. S., and Boggan, W. O. (1977) Monoamine oxidase inhibition in brain and liver produced by d-carbolines Structure-activity relationships and substrate specificity. Biochem. Pharmacol., 26 1991-1996. 

It was reported that the distribution and activities of esterases that catalyze pyrethroid metabolism using several human and rat tissues, including small intestine, liver, and serum, were examined [30]. The major esterase in human intestine was hCE2. //c/n.v-Permethrin was effectively hydrolyzed by pooled human intestinal microsomes (five individuals), while deltamethrin and bioresmethrin were not. This result correlated well with the substrate specificity of recombinant hCE2. In contrast, pooled rat intestinal microsomes (five animals) hydrolyzed trans-permethrin 4.5 times slower than the human intestinal microsomes. Furthermore, pooled samples of cytosol from human or rat liver were ca. half as hydrolytically active as the corresponding microsome fraction toward pyrethroids however, the cytosolic fractions had significant amounts (ca. 40%) of the total hydrolytic activity. Moreover, a sixfold interindividual variation in hCEl protein expression in human hepatic cytosols was observed. 

Fourth, most esterases are highly polymorphic enzymes. Many of the purified carboxylesterases are mixtures of isoenzymes that have different substrate specificities [60][61]. For example, the substrate(s) used to isolate pig liver carboxylesterase influences the isoenzyme composition, and, hence, the substrate specificity of the resulting esterase preparation. 

A number of rat liver carboxylesterases identified by their pI values are listed in Table 2.6 [73] five nonspecific carboxylesterases were purified from rat liver and were characterized according to their p/ values [61]. They appeared to be isoenzymes, since they had similar substrate specificities toward phenyl and naphthyl esters and monooleylglycerol. Subsequent studies, however, revealed different specificities with respect to their physiological substrates. The pI 5.2 and 5.6 enzymes were shown to be acylcamitine hydrolases (EC 3.1.1.28), and a p/ 6.0 enzyme an octanoylglycerol lipase. The p/... 

Three isoenzymes of carboxylesterase were purified from rat liver micro-somes and were named RL1, RL2, and RH1. These differ from each other in their response to hormone treatment, inducibility, substrate specificity, and immunological properties [75], It was shown that RL1, RL2, and RH1 resemble hydrolases p/ 6.2/6.4, pI 6.0, and pI 5.6, respectively. Enzyme RL2 was found to be identical to egasyn, a protein with esterase activity found in the endoplasmic reticulum [76], The role of egasyn is to stabilize glucuronidase (EC 3.2.1.31) by noncovalent binding to the microsomal membrane. 

Series of homologous esters have been investigated to try to establish structure-metabolism relationships, however partial and limited the latter may be. This aspect will be discussed again in the context of prodrugs (Chapt. 8). Here, we mention a few representative studies in which model substrates were used. Table 7.2 documents the substrate specificity of a rabbit liver carboxylesterase (ES-1A) toward homologous series of methyl, 4-nitrophenyl, a-naphthyl, /1-naphthyl, and 4-methylumbelliferyl esters [41]. In... 

Andersson U, Eggertsen G, Bjorkhem I. 1998. Rabbit liver contains one major sterol 12alpha-hydroxylase with broad substrate specificity. Biochim Biophys Acta 1389 150-154. 

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