Complex sentence: In almost all land animals, urate oxidase further catalyzes the oxidation of uric acid to allantoin, but in humans and most higher primates, the urate oxidase gene is nonfunctional, so that UA is not further broken down.
Split into simpler sentences: In almost all land animals, urate oxidase further catalyzes the oxidation of uric acid to allantoin. However, in humans and most higher primates, the urate oxidase gene is present, but nonfunctional, so that UA is not further broken down.