title: Scavenging properties of neutrophil 4-hydroxyphenylpyruvate dioxygenase are based on a hypothesis that does not stand up to scrutiny.
abstract: It was previously reported by D'Eufemia et al. [9] that neutrophil preparations from a patient with tyrosinemia type III, i.e. with inherited deficiency of 4-hydroxyphenylpyruvate dioxygenase (HPPD), exhibited a far higher NO release than controls, when NO was estimated in terms of nitrite content in the suspending media. It was hypothesized that HPPD might participate to NO sequestration in neutrophils and that excessive NO release might reflect the lack of the scavenging action in defective cells. In recent control experiments, we found that HPPD activity in neutrophils preparations from healthy subjects is below the detection limit of the enzymatic assay (less than 3nmol product/h per mg protein). This indicates that HPPD concentration in neutrophils is very low, if any, confirming what was already suggested in literature, and rules out the possibility of a prominent role of HPPD as NO scavenger in these cells. Moreover, we found that 500μM l-tyrosine increases nitrite release and accumulation in suspending media of U-937 cells, a human monoblast-like lymphoma cell line which displays many characteristics of macrophages, including the expression of inducible and endothelial nitric oxide synthases. We hypothesize that the increase of nitrite release by patient's neutrophils might be related to the presence of high l-tyrosine concentrations in the blood samples (426μmol/L instead of 52.1±10.9μmol/L as healthy subjects), rather than to HPPD deficiency of in these cells.
authors.affiliation: Department of Biochemical Sciences, University of Roma La Sapienza, 00161 Rome, Italy. Electronic address: costantino.salerno@uniroma1.it.
authors.forename: Costantino
authors.initials: C
authors.lastname: Salerno
authors.affiliation: Department of Experimental Medicine, University of Roma La Sapienza, 00161 Rome, Italy.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Zicari
authors.affiliation: Department of Experimental Medicine, University of Roma La Sapienza, 00161 Rome, Italy.
authors.forename: Emanuela
authors.initials: E
authors.lastname: Mari
authors.affiliation: Department of Pediatrics, University of Roma La Sapienza, 00161 Rome, Italy.
authors.forename: Patrizia
authors.initials: P
authors.lastname: D'Eufemia
mesh_terms: D015111:4-Hydroxyphenylpyruvate Dioxygenase / Q000378:metabolism*; D016166:Free Radical Scavengers / Q000378:metabolism*; D006801:Humans; D009504:Neutrophils / Q000187:drug effects / Q000201:enzymology*; D014443:Tyrosine / Q000494:pharmacology; D020298:U937 Cells
chemical_list: D016166:Free Radical Scavengers; D014443:Tyrosine; D015111:4-Hydroxyphenylpyruvate Dioxygenase
keywords: Nitric oxide; Tyrosinemia type III; Tyrosyl radicals