Steroid 5a reductases

Sodium, potassium ATPase HIV reverse transcriptase Steroid 5a-reductase... 

Finasteride, epristeride, Steroid 5a-reductase Benign prostate... 

We saw in Chapter 3 that bisubstrate reactions can conform to a number of different reaction mechanisms. We saw further that the apparent value of a substrate Km (KT) can vary with the degree of saturation of the other substrate of the reaction, in different ways depending on the mechanistic details. Hence the determination of balanced conditions for screening of an enzyme that catalyzes a bisubstrate reaction will require a prior knowledge of reaction mechanism. This places a necessary, but often overlooked, burden on the scientist to determine the reaction mechanism of the enzyme before finalizing assay conditions for HTS purposes. The importance of this mechanistic information cannot be overstated. We have already seen, in the examples of methotrexate inhibition of dihydrofolate, mycophenolic acid inhibiton of IMP dehydrogenase, and epristeride inhibition of steroid 5a-reductase (Chapter 3), how the [5]/A p ratio can influence one s ability to identify uncompetitive inhibitors of bisubstrate reactions. We have also seen that our ability to discover uncompetitive inhibitors of such reactions must be balanced with our ability to discover competitive inhibitors as well. 

Xanthine oxidase (1-Lactamase Ornithine decarboxylase Steroid 5a-reductase Thymidylate synthase Aromatase... 

Our second example of drugs that function as mechanism-based inactivators is the steroid 5a-reductase inhibitors finasteride and dutasteride. The mechanism of inactivation by these compounds is an interesting departure from the typical target enzyme covalent modification seen with most mechanism-based inactivators. 

Evidence that BPH could be hormone related came from studies of a population of pseudohermaphrodites in the Dominican Republic. These individuals are genetically male, but do not display normal male genitalia until the onset of puberty. They are therefore raised as females until puberty. Studies revealed that these pseudohermaphrodites are deficient in an isoform of the enzyme steroid 5a-reductase, which is responsible for catalyzing the conversion of testosterone to dihydrotestosterone (DHT). In addition to the overt sexual manifestations of this condition, affected individuals show no incident of male pattern baldness, mild or no acne, and underdevelopment of the prostate. These observations led researchers to postulate that a selective inhibitor of steroid 5a-reductase would be an effective treatment for BPH. 

The reaction catalyzed by steroid 5a-reductase is illustrated in Figure 8.17 (Harris and Kozarich, 1997). The reaction follows a compulsory ordered ternary... 

Figure 8.17 Reaction mechanism of testosterone reduction to dihydrotestosterone (DHT) as catalyzed by the enzyme steroid 5a-reductase.

Finasteride (Figure 8.18A) was designed as a mimic of the substrate testosterone. Preliminary studies suggested that the compound was a slow, tight binding reversible inhibitor of steroid 5a-reductase. A combination of detailed kinetic and... 

Figure 8.18 Mechanism-based inactivators of steroid 5a-reductase. (A) Finasteride, (B) the bisubstrate analogue formed by reaction of NADP1 with finasteride catalyzed by the enzyme, and (C) dutasteride.

Finasteride has been clinically proved to reduce the median volume of the prostate in patients and is currently prescribed for the treatment of BPH. The compound also has demonstrated efficacy in the treatment of male pattern baldness and is prescribed for this indication as well. Subsequent to the discovery of finasteride, it was found that there are two isoforms of steroid 5a-reductase in mammals, type 1 and type 2. The type 2 isoform is primarily active in reproductive tissue, while the type 1 isoform contributes to DHT formation in the skin, liver, and reproductive tissue. Finasteride inhibits both isozymes in rats, but selectively inhibits the type 2 isozyme only in humans. It is hypothesized that dual inhibition of both isoforms of steroid 5a-reductase might prove more effective in treating BPH. Hence the GlaxoSmithKline group identified and developed dutasteride (Figure 8.18C). Dutasteride inactivates both human isoforms of steroid 5a-reductase by a mechanism similar to that described for finasteride (Bramson et al., 1997 see also the Web site www.avodart.com). Both finasteride and dutasteride have demonstrated clinical efficacy and are currently used in the treatment of BPH. 

External genitalia of men deficient in steroid 5a-reductase are female In character rather than male. 

Males with deficiency of the 5a-reductase isoenzyme do not develop acne, male pattern baldness, or enlarged prostates.274 The last fact was some of the impetus for development of the steroid 5a-reductase inhibitor finasteride, which is widely used to treat benign prostate enlargement 274/277/278 It is an enzyme-activated inhibitor in which the NADH reduces the C= C bond in the A ring, which is not in the same position as in the substrate. The resulting anion cannot become protonated but instead adds to the NAD+ as shown in Eq. 22-15. 

Avodart1M Dutasteride 0.5 mg SEC Benign prostatic hyperplasia A selective inhibitor of both the type 1 and type 2 isoform of steroid 5a-reductase Mono-di-glycerides of caprylic/ capric acid, butylated hydroxytoluene Glaxosmithkline (GSK)... 

Figure 1736 Inhibition of steroid 5a-reductase by finasteride (PADPR = phosphoadenosine dipho sphoribo se).

Liao, S. and Hiipakka, R. A. 1995. Selective inhibition of steroid 5a-reductase isozymes by tea epicatechin-3-gallate and epigallocatechin-3-gallate. Biochem. Biophys. Res. Commun. 214 833-38. 

Anderson, S. and Russell, D. W. 1990. Structural and biochemical properties of cloned and expressed human and rat steroid 5a-reductase. Proc. Natl. Acad. Sci. USA 87 3640-44. 

At many sites of action, testosterone is not the active form of the hormone. It is converted by steroid 5a-reductases in target tissues to the more active dihydrotestosterone. Steroid 5a-reductase 1 is located largely in nongenital skin and liver, and steroid 5a-reductase 2 is present principally in the urogenital tract of the male and in the genital skin of both sexes. 

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