Isocitrate lyase enzyme is used by plants, bacteria and fungi that catalyze the cleavage of isocitrate to succinate and glyoxylate in the glyoxylate cycle. And jointly with maltase synthase it avoids the two decarboxylation steps of the TCA (Tri carboxylic acid) cycle.
A mutant has no activity on the enzyme isocitrate lyase does not prove that the mutation is in the gene encoding isocitrate lyase, because the enzyme might need post-translational modification to become active.