Horse-liver alcohol dehydrogenase and Pseudomonas testosteroni 3(17)beta-hydroxysteroid dehydrogenase transfer epimeric hydrogens from NADH to 17beta-hydroxy-5alpha-androstan-3-one. An exception to one of the Alworth-Bentley rules.

This study observed an exception to a widely accepted rule regarding the stereospecificity of enzymes in their reactions with hydrogen particles. The researchers found that two different enzymes, horse liver alcohol dehydrogenase and Pseudomonas testosteroni 3(17)beta-hydroxysteroid dehydrogenase, react differently to the same substrate, 17beta-hydroxy-5alpha-androstan-3-one, by using different hydrogen particles.

Research Context

• Alworth and Bentley had previously proposed a rule stating that the stereospecificity of an enzymatic reaction with the hydrogen particles at C-4 of NADH and NADPH is fixed and does not change depending on the enzyme used.

Methodology

• In order to challenge this rule, the researchers observed the reaction of two different enzymes, horse liver alcohol dehydrogenase and Pseudomonas testosteroni 3(17)beta-hydroxysteroid dehydrogenase, with the same substrate, 17beta-hydroxy-5alpha-androstan-3-one.

Results

• They found that horse liver alcohol dehydrogenase uses the 4-pro-R hydrogen of NADH in its reaction with the substrate.
• On the other hand, Pseudomonas testosteroni 3(17)beta-hydroxysteroid dehydrogenase used the 4-pro-S hydrogen in its reaction.

Conclusion

• These findings provide a clear exception to the Alworth-Bentley Rule, suggesting that the stereospecificity of an enzymatic reaction can be affected by the source of the enzyme preparation.
• This work also challenges the idea that the “best fit” of substrate and cofactor is what dictates the side of NADH from which the hydride is transferred to the substrate. They point out that in the case of these two enzymes, this cannot be the case, given their different reactions.