This is because one of the enzymes required for its use is an oxidase ( requiring molecular oxygen ) .
The other is repressed by the presence of ammonium ( another source of assimilable nitrogen needed by yeast ) in the must .
This is because one of the enzymes required for its use is an oxidase ( requiring molecular oxygen ) and the other is repressed by the presence of ammonium ( another source of assimilable nitrogen needed by yeast ) in the must .