Science Buddies: "Ask an Expert"

I have a very specific question about enzymes please.
I read it in several sources that HSDs, (Hydroxysteroid Dehydrogenases) are a family of ARKs (Aldo-keto Reductases).
This makes no sense to me because a dehydrogenase is an enzyme that catalyzes the removal of hydrogen from a substrate, and therefore oxidizes it, while an ARK is an enzyme that catalyzes the addition of hydrogen to a keton carbon-oxygen bond.
How can these be within the same family?

Both reactions occur within the enzyme. In order to reduce an aldehyde or a ketone in the steroid, you need to grab a "hydride" from somewhere else, such as an NADH or NADPH. See figure 1 in this article:

http://www.danforthcenter.org/jez/pdf/1 ... 201998.pdf

The accompanying text describes it as well. From the point of view of the steroid, it is a reductase. From the point of view of NADH or NADPH, it is a dehydrogenase.

Let me know if this doesn't answer your question, or if you can't get to the above link.

Thank you very much. It actually did address my question very well.