Cytosolic hydrolase metabolism

Potential enzymes involved in anthocyanin metabolism — The lactase phlorizin hydrolase (LPH EC 3.2.1.108) present only in the small intestine on the outside of the brush border membrane and the cytosolic P-glucosidase (CBG EC 3.2.1.1) found in many tissues, particularly in liver, can catalyze the deglycosylation (or hydrolysis) of polyphenols. LPH may play a major role in polyphenol metabolism... 

The overall metabolism of vitamin A in the body is regulated by esterases. Dietary retinyl esters are hydrolyzed enzymatically in the intestinal lumen, and free retinol enters the enterocyte, where it is re-esterified. The resulting esters are then packed into chylomicrons delivered via the lymphatic system to the liver, where they are again hydrolyzed and re-esterified for storage. Prior to mobilization from the liver, the retinyl esters are hydrolyzed, and free retinol is complexed with the retinol-binding protein for secretion from the liver [101]. Different esterases are involved in this sequence. Hydrolysis of dietary retinyl esters in the lumen is catalyzed by pancreatic sterol esterase (steryl-ester acylhydrolase, cholesterol esterase, EC 3.1.1.13) [102], A bile salt independent retinyl-palmitate esterase (EC 3.1.1.21) located in the liver cell plasma hydrolyzes retinyl esters delivered to the liver by chylomicrons. Another neutral retinyl ester hydrolase has been found in the nuclear and cytosolic fractions of liver homogenates. This enzyme is stimulated by bile salts and has properties nearly identical to those observed for... 

D. C. Zeldin, S. Wei, J. R. Falck, B. D. Hammock, J. P. Snapper, J. H. Capdevilla, Metabolism of Epoxyeicosatrienoic Acids by Cytosolic Epoxide Hydrolase Substrate Structural Determinants of Asymmetric Catalysis , Arch. Biochem. Biophys. 1995, 316, 443 - 451. 

Since liver is the most important organ for metabolism investigations the procedures described here focus on liver cytosol exemplarily. Liver cytosol fraction contains soluble Phase I and Phase II enzymes which play an important role in drug metabolism (Brandon 2003). These are alcohol and aldehyde dehydrogenases, epoxide hydrolases, sulfotransferases, glutathione S transferase, N-acetyl transferases, and methyl transferases. Therefore, in cytosolic preparations these biotransformation steps can be studied. Cytosolic fractions are commercially available (BDGentest, Invitro Technologies, Xenotech and others) or easy to prepare, alternatively. 

There are two distinct types of epoxide hydrolases, both widely distributed in mammalian tissues. One type is localized primarily in the endoplasmic reticulum, the second in the cytosol. The microsomal and cytosolic enzymes have different properties, including substrate selectivities. Several inducers of xenobiotic metabolizing enzymes, including pheno-barbital, planar PCB congeners, and ra s-stilbene oxide, selectively increase (induce) microsomal, but not cytosolic, epoxide hydrolase activity. 

The role of the APS sulfohydrolases in sulfate metabolism is not understood precisely. The lysosomal APS sulfohydrolase can hydrolyze bis(4-nitro-phenyl) phosphate and 4-nitrophenyl 5 -phosphothymidine (Roger et al., 1978). Thus the lysosomal APS sulfohydrolase is less specific than its cytosolic counterpart, which does not hydrolyze these nitrophenyl derivatives. The apparent role of the lysosomal enzyme is to hydrolyze the acid anhydrides of such compounds as FAD, ATP, and ADP in secondary lysosomes. Thus lysosomal APS sulfohydrolase is an acid anhydride hydrolase that helps the cell in the recovery of nucleoside monophosphates from acid anhydrides. The APS sulfohydrolase in the cytosolic fraction probably regulates the concentrations of PAPS and therefore plays an important role in the control of sulfate conjugation. 

Epoxidation by the introduction of an oxygen atom into flunarizine produced l-[bis(4-fluoro-phenyl)methyl] - 4 - [(3-phenyloxiran - 2 - yl)methyl] piperazine (metabolite 2) and epoxide hydration to a diol, 3-[4-[bis(4-fluorophenyl)methyl]-l-piper-azinyl]-l-phenyl-l,2-propanediol (metabolite 10). Lavrijsen et al. (1992) found metabolites formed by epoxidation at the double bond (metabolite 2) and epoxide hydration (metabolite 10) in incubates with subcellular hepatocyte fractions of male and female rats. Metabolites formed by epoxidation and epoxide hydration were not detected in vivo (Meuldermans et al. 1983), probably because the resulting metabolites were metabolised in vivo, much more quickly than in vitro, into secondary metabolites. A diol metabolite, however, was described for the metabolism of l-butyl-4-dimamyl-piperazine in guinea pigs (Morishita et al. 1978). With supernatant fractions a rapid disappearance of the epoxide intermediate from incubate was observed. This seems to indicate that, for the epoxide hydrolysis, besides microsomal epoxide hydrolase, cytosolic epoxide hydrolase might also be involved. 

Fig. 79.5 Hypothetic pathways of anthocyanin absorption, distribution, metabolism, and excretion based on current information (CGB-cytosolic beta-glucosidase SULT-sulfotransferase UDP-GT-glucuronosyltransferase COMT-catechoi-O-methyl transferase SGLT-sodium-dependent glucose cotransporters LPH-lactase-phlorizin hydrolase)...

Figure 8.3 A schematic representation of the general mechanisms of flavonoid absorption, metabolism, and elimination using quercetin glycosides and (-l-)-catechin as examples. MRP, multidrug resistance protein CBG, cytosolic P-glucosidase LPH, lactase phloridzin hydrolase SGLT, sodium-dependent glucose transporter.

---