HHH syndrome results from the defect in the enzyme is

Ornithine PermeaseThe hyperornithinemia, hyperammonemia, and homocitrullinuria syndrome (HHH syndrome) result from mutation of the ORNT1 gene that encodes the mitochondrial membrane ornithine permease. The failure to impo cytosolic ornithine into the mitochondrial matrix renders the urea cycle inoperable, with consequent hyperammonemia, and hyperornithinemia due to the accompanying accumulation of cytosolic ornithine. In the absence of its normal acceptor (ornithine), mitochondrial carbamoyl phosphate carbamoylates lysine to homocitrulline, resulting in homocitrullinuria.Ref: Harper&;s Biochemistry; 30th edition; Chapter 28; Catabolism of Proteins & of Amino Acid Nitrogen