Human Carbonyl Reduction Pathways and a Strategy for Their Study In Vitro

Abstract

Carbonyl reduction plays a significant role in physiological processes throughout the body. Although much is known about endogenous carbonyl metabolism, much less is known about the roles of carbonyl-reducing enzymes in xenobiotic metabolism. Multiple pathways exist in humans for metabolizing carbonyl moieties of xenobiotics to their corresponding alcohols, readying these molecules for subsequent conjugation and/or excretion. When exploring carbonyl reduction clearance pathways for a drug development candidate, it is possible to assess the relative contributions of these enzymes due to their differences in subcellular locations, cofactor dependence, and inhibitor profiles. In addition, the contributions of these enzymes may be explored by varying incubation conditions, such as pH. Presently, individual isoforms of carbonyl-reducing enzymes are not widely available, either in recombinant or purified form. However, it is possible to study carbonyl reduction clearance pathways from simple experiments with commercially available reagents. This article provides an overview of carbonyl-reducing enzymes, including some kinetic data for substrates and inhibitors. In addition, an experimental strategy for the study of these enzymes in vitro is presented.

Pharmacogenetics of Human Carbonyl Reductase 1 (CBR1) in Livers from Black and White Donors

“…The reduction of carbonyl groups by hepatic CBR1 activity is an important step during the metabolism of clinically relevant drugs such as haloperidol (antipsychotic), doxorubicin (anticancer), dolasteron (antiemetic), and pentoxyfilline (hemorheologic) (Rosemond and Walsh, 2004). On the other hand, differences in the average expression of drug-metabolizing enzymes between ethnic groups with distinctive geographical ancestries may affect drug response and toxicity (Wilson et al, 2001;Daar and Singer, 2005;Diczfalusy et al, 2008).…”

“…The aldo-keto reductases AKR1A1 and AKR1B1 are expressed in liver and also catalyze the reduction of doxorubicin. However, AKR1A1 and AKR1B1 have 7-to 18-fold lower catalytic efficiencies for the reduction of anthracycline substrates compared with CBR1 (Wermuth et al, 1986;Ohara et al, 1995;O'connor et al, 1999;Rosemond and Walsh, 2004;Kassner et al, 2008). Thus, variable hepatic CBR1 expression may affect the unpredictable pharmacodynamics of doxorubicin.…”

“…Carbonyl reductase 1 (CBR1) is a cytosolic short-chain dehydrogenase that catalyzes the two-electron reduction of relevant pharmacological substrates such as the antipsychotic haloperidol and the anticancer anthracyclines doxorubicin and daunorubicin (Forrest and Gonzalez, 2000;Rosemond and Walsh, 2004;Matsunaga et al, 2006). CBR1 reduces doxorubicin into its main circulating C-13 alcohol metabolite doxorubicinol (doxol) by using the cofactor NADPH (Wermuth, 1981;Forrest et al, 1990).…”

Carbonyl Reductase 1 Is a Predominant Doxorubicin Reductase in the Human Liver

“…Considered together with its size and the high DOX intrinsic clearance by the hepatic cytosol (Table 1), liver appears to be the most important organ in DOX metabolism, followed by kidney and the gastrointestinal tract, represented by stomach and colon. Of the known NADPH-dependent, cytosolic CBRs (Rosemond and Walsh, 2004), all but one (xylulose reductase) were investigated in the present study in the form of recombinant enzymes as candidates for the hepatic DOX reductase. Altogether, six enzymes (CBR1, AKR1A1, AKR1B1, AKR1B10, AKR1C3, and AKR1C4) were capable of DOX reduction.…”

“…However, neither detailed kinetics nor the individual importance of these enzymes in the reduction of DOX to DOX-OL, nor the existence of additional ones, is known. Therefore, we investigated the kinetics of DOX reduction of seven cytosolic AKRs and of two CBRs because these enzyme families catalyze cytosolic carbonyl reduction (Rosemond and Walsh, 2004). Besides conflicting results obtained in heart cytosols (Mordente et al, 2003;Salvatorelli et al, 2006), kinetic data of DOX reduction in human tissues are available only for a single sample of liver and kidney (Lovless et al, 1978).…”