A defect in molybdenum cofactor binding causes an attenuated form of sulfite oxidase deficiency

Abstract

Isolated sulfite oxidase deficiency (ISOD) is a rare recessive and infantile lethal metabolic disorder, which is caused by functional loss of sulfite oxidase (SO) due to mutations of the SUOX gene. SO is a mitochondrially localized molybdenum cofactor (Moco)-and heme-dependent enzyme, which catalyzes the vital oxidation of toxic sulfite to sulfate. Accumulation of sulfite and sulfite-related metabolites such as S-sulfocysteine (SSC) are drivers of severe neurodegeneration leading to early childhood death in the majority of ISOD patients. Full functionality of SO is dependent on correct insertion of the heme cofactor and Moco, which is controlled by a highly orchestrated maturation process. This maturation involves the translation in the cytosol, import into the intermembrane space (IMS) of mitochondria, cleavage of the mitochondrial targeting sequence, and insertion of both cofactors. Moco insertion has proven as the crucial step in this maturation process, which enables the correct folding of the homodimer and traps SO in the IMS. Here, we report on a novel ISOD patient presented at 17 months of age carrying the homozygous mutation NM_001032386.2 (SUOX): c.1097G > A, which results in the expression of SO variant R366H. Our studies show that histidine substitution of Arg366, which is involved in coordination of the Moco-phosphate, causes a severe reduction in Moco insertion efficacy in vitro and in vivo. Expression of R366H in HEK SUOX À/À cells mimics the phenotype of patient's fibroblasts, representing a loss of SO expression and specific activity. Our studies disclose a general paradigm for a kinetic defect in Moco insertion into SO caused by residues involved in Moco coordination resulting in the case of R366H in an attenuated form of ISOD.

Molybdenum Cofactor Deficiency in Humans

“…Biochemical characterization of those patients’ SOX variants (G362S, R366H) revealed an impairment in Moco coordination within the active site of SOX, resulting in strongly decreased mitochondrial maturation. Following the culture of patient fibroblasts in the presence of molybdate [ 96 ] or MOCS1 transgene expression [ 98 ], significantly increased SOX activity has been observed, thus demonstrating that the increase in Moco availability partially rescues a defect in Moco-dependent SOX maturation. Therefore, the molecular–genetic investigation of iSOD patients may identify future molybdate-responsive patients, opening the path to personalized medicine.…”

Differentiation of HSA and BSA and Instantaneous Detection of HSO3– Using Confined Space of Serum Albumins and Live Cell Imaging of Exogenous/Endogenous HSO3–

“…In physiological conditions, sulfites are formed by the reaction of SO 2 and water; SO 2 + H 2 O ↔ H 2 SO 3 – ↔ HSO 3 – + H + ↔ SO 3 2– + 2H + . The mitochondrial enzyme sulfite oxidase catalyzes the oxidative degradation of cysteine and methionine and also plays a vital role in detoxifying exogenously supplied sulfite from food, pharmaceutical products, and antimicrobial agents. , However, the deficiency in the sulfite oxidase enzyme or the error in the biosynthetic pathway results in the accumulation of sulfite in biological fluids or tissues. − The accumulation of sulfite affects the mitochondrial homeostasis in rat brain mitochondria and also induces mitochondrial swelling and reduces the mitochondrial membrane potential and Ca 2+ retention capacity . Bisulfite is associated with respiratory abnormalities and brain cancer .…”

“… 30 , 31 However, the deficiency in the sulfite oxidase enzyme or the error in the biosynthetic pathway results in the accumulation of sulfite in biological fluids or tissues. 32 − 34 The accumulation of sulfite affects the mitochondrial homeostasis in rat brain mitochondria and also induces mitochondrial swelling and reduces the mitochondrial membrane potential and Ca 2+ retention capacity. 35 Bisulfite is associated with respiratory abnormalities 36 and brain cancer.…”