Complex sentence: Addition of carbonyl compounds to the enzyme, in the presence of hydrogen cyanide, increases hydrogen cyanide's ability to inhibit acetoacetate decarboxylase suggesting that carbonyl compounds readily form Schiff's bases at the active site.
Split into simpler sentences: Addition of carbonyl compounds to the enzyme, in the presence of hydrogen cyanide, increases hydrogen cyanide's ability to inhibit acetoacetate decarboxylase. This is suggestive of the carbonyl compounds ability to readily form Schiff's bases at the active site.