What They Saw
They measured activity of purified enzyme with an electrode measuring hydrogen oxidation in the presence of methylene blue dye (an electron acceptor).
They measured stability of the enzyme in the presence of oxygen, and found that crude extracts were very stable (and could go for weeks without losing activity), but the more pure the preparation, the less oxygen-tolerant it was: the most pure lost half its activity in 20 minutes at 20% oxygen. This inactivation was irreversible.
With a very good electron donor (methyl viologen), the hydrogenase could produce hydrogen. The highest rate they saw was 3.4 μmol hydrogen per minute per mg protein, which peaked at a fairly low pH (around 4). The rate was almost 0 closer to neutral. Also, the presence of hydrogen in the environment inhibits its production by the enzyme.
Even when electron acceptors were not present, the enzyme could combine one deuterium from D2 with one hydrogen from water to make HD.
The enzyme seemed to be good at donating electrons to acceptors with positive mid-point potentials but not to negative ones, so it seems to have a higher potential than reversible hydrogenases.