2- Hydroxylation pathway

In the non-3-hydroxylation pathway, the initial step from GA] 2 alenzyme-catalyzed oxidation of carbon-7 which yields the acid, GAqg. This is followed by the loss of carbon-20 to give GAq. The mechanism for the loss of carbon-20 is still unresolved. There are several lines of evidence which eliminate a number of possible mechanisms. For instance the two oxygens in the lactone of GAq have been shown to have their origin from the 19-oic acid group of the Cgo precursor (25). 

GA36 and GA 3, have been shown to be metabolites from radio-labeled feeds of GA]J+ to the fungus (2 ). Thus the mechanism for the loss of carbon-20 is also unresolved in the early-3-hydroxylation pathway. 

GA] 3 (named GA3g), and the C g gibberellins, GA (named 6A5g), and GA3 (named GA g). Thus they are potential members of an early-12o-hydroxylation pathway analogous in oxidation and hydroxylation pattern to the early-3-hydroxylation pathway alternatively, they may be a series of single step metabolites that branch from an early-3-hydroxylation pathway. Recent and extensive radiolabeled feeds to cell-free systems from pumpkin suggest both possibilities (, ). In this system the conversions are highly... 

EARLY-3-HYDROXYLATION PATHWAY FUNGUS HIGHER PLANTS... 

These results indicate that in the in vitro reaction, the a-hydroxylation pathway accounts for about 34% of the metabolism of DMN and about 19% of the metabolism of NMA, when uninduced Fisher 344 rat liver S-9 is used Thus, our data for DMN fall roughly in-between the two previously published... 

Fiore, A. et al.. Elucidation of the b-carotene hydroxylation pathway in Arabidopsis thahana, FEBS Lett. 580, 4718, 2006. 

Methyl-substituted indole has been the subject of an investigation and the degradation metabolites were identified [346], An indole-degrading methanogenic consortium induced a two-step reaction on 3-methylindole, through a hydroxylation pathway that... 

The reactive mutagens formed from arylamides are hydroxylamines (Fig. 4.8) produced either by deacetylation and subsequent (V-hydroxylation (Pathway a), or by A-hydroxylalion and subsequent deacetylation (Pathway b). An alternative pathway to form A-hydroxyaminofluorene involves transacetylation by A, 0 - acy I Iran sI erase, producing A-acetoxyarylamine (Pathway c). The latter mode of activation seems to operate in human hepatocytes, since... 

Fig. 4.8. Formation of mutagenic N-hydroxyamines from arylamides. Pathway a via deacetylation and subsequent IV-hydroxylation. Pathway b via IV-hydroxylation and subsequent deacetylation. Pathway c via N-acetoxy arylamine produced by IV,0-acyltransferases. [99]. Activation of hydroxylamines and hydroxylamides by O-sulfation is not shown. In all cases, the ultimate electrophile may be a nitrenium ion.

Metabolism/Excretion - There are 2 genetically determined patterns of propafenone metabolism. In more than 90% of patients, the drug is rapidly and extensively metabolized with an elimination half-life of 2 to 10 hours. These patients metabolize propafenone into two active metabolites 5-hydroxypropafenone and N-depropylpropafenone. They both are usually present in concentrations less than 20% of propafenone. The saturable hydroxylation pathway is responsible for the nonlinear pharmacokinetic disposition. 

According to Barcza127, the skeleton of 6,6-dimethyl-6-sila-steroids could have interesting properties because the steroid 6-hydroxylation pathway would be blocked in such structures, and aromatization of ring B would be made impossible because the formation of a stable Si-C double bond is not possible. The sila-sub-stitution in the middle of a polycondensed ring skeleton therefore prevents the eventual formation of polycondensed aromaticity avoiding possible carcinogenic effects. 

Figure 2 Consensus mechanism for fatty acid desaturation. The related minor hydroxylation pathway also is shown.

Perhaps the most important mechanistic question that remains unanswered with respect to desaturases relates to the switch that controls the choice of dehydrogenation/hydroxylation pathways (Fig. 2). The study of the EAD2 subgroup of plant desaturases has been particularly instructive in this context. Pairs... 

FIGURE 30.13 Observed total elimination clearance (CLg) of midazolam (MDZ) in liver transplant patients versus hepatic clearance (CLjj) of 1 -OH MDZ predicted from biopsy specimens. The line is the predicted hepatic clearance of MDZ if the I -hydroxylation pathway accounts for 70% of the substrate loss. (Data from Thummel KE et al. J Pharmacol Exp Ther 1994 271 549-56.)... 

Vmax and Km for MDZ 1 -liydroxylation. V,nax determined for each biopsy sample was then scaled to the estimated total liver mass and intrinsic clearance estimated as total liver Vmax/Kn- Hepatic clearance then was predicted from Equation 7.6 in Chapter 7. Figure 30.13 compares the observed total elimination clearance with predicted hepatic clearance based on the assumption that the E-hydroxylation pathway accounts for 70% of the substrate loss. The prediction is quite good. The average absolute deviation between the five observed data points and their predicted values is only 28% and the differences are uniformly distributed. 

The insertion of hydroxyl groups into the 23- or 24-position of 5P-cholestane-3a,7a,12a,25-tetrol was found to be stereospecific. Although all these compounds were potential precursors of bile acid, studies in vivo and in vitro experiments using [3P- H] and (24- C) 5P-cholestane-3a,7a,12a,25-tetrol (46) (Figs.6, 7), (24- C) 5p-cholestane-3a,7a,12a,24R,25-pentol and (24- C) 5P-cholestane-3a,7a,12a,24S,25-pentol demonstrated the existence of a new 25-hydroxylation pathway for the transformation of cholesterol to cholic acid in these patients (2,10). The reaction sequence involved the stereospecific formation of a 24S-hydroxy pentol, 5P-cholestane-3a,7a,12a,24S,25-pentol, 3a7a,12a,25-tetrahydroxy-5P-cholestan-24-one and did not involve SP-cholestanoic acids as intermediates (Fig. 8). The two bile pentols, SP-cholestane-3a,7a,12a,24R, 25-pentol and 5P-cholestane-3a,7a,12a,23R,25-... 

In summary, these studies demonstrated that in CTX the impaired synthesis of bile acids is due to a defect in the biosynthetic pathway involving the oxidation of the cholesterol side-chain. As a consequence of the inefficient side-chain oxidation, increased 23, 24 and 25-hydroxylation of bile acid precursors occurs with the consequent marked increase in bile alcohol glucuronides secretions in bile, urine, plasma and feces (free bile alcohols). These compounds were isolated, synthesized and fully characterized by various spectroscopic methods. In addition, their absolute stereochemistiy determined by Lanthanide-Induced Circular Dichroism (CD) and Sharpless Asymmetric Dihydroxylation studies. Further studies demonstrated that (CTX) patients transform cholesterol into bile acids predominantly via the 25-hydroxylation pathway. This pathway involves the 25-hydroxylation of 5P-cholestane-3a,7a, 12a-triol to give 5P-cholestane-5P-cholestane-3a,7a,12a,25- tetrol followed by stereospecific 24S-hydroxylation to yield 5P-cholestane-3a,7a,12a,24S,25-pentol which in turn was converted to cholic acid. 

An example of this hydroxylation pathway is seen in the metabolism of the oral hypoglycemic agent acetohexamide... 

Direct N-dealkylation of r-butyl groups, as discussed above, is not possible by the a caitxin hydroxylation pathway. In vitro studic.s indicate, however, that N-/-bulylnor-... 

Metabolism of the important cancer chemotherapeutic agent cyclophosphamide (Cytoxan) follows a hydroxylation pathway similar to that Just described for cyclic amides. This drug is a cyclic phosphoramide derivative and, for the motit part, is the phosphorous counterpart of a cyclic amide. Because cyclophosphamide itself is pharmacologically inac-... 

Small amounts of phenytoin are excreted unchanged in the urine (2-4%) and feces (5%). Most is eliminated renally as inactive conjugated metabolites. The elimination half-life at linear doses averages 20-30 h (12-20 h in children) but may be as long as 60 h, and as high as 200 h after overdose, due to saturation of hydroxylation pathways. The maximum rate of metabolism is estimated at 6mgkg day. ... 

Ethotoin. Chemically, 3-ethyl-5-phenylhy-dantoin, ethotoin (Ic) undergoes two biotransformation pathways leading to inactive products p-hydroxylation [pathway (1)] and deethylation [pathway (2)]. This product has relatively low potency compared to that of phenytoin. Like phenytoin, ethotoin displays saturable metabolism with respect to the formation of the two metabolites (18). 

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