Relación estructura-función de la enzima uroporfirinógeno descarboxilasa: implicancias para la comprensión y el tratamiento de la porfiria cutánea tarda / Structure-function relationship of the enzyme uroporphyrinogen decarboxylase: implications for the understanding and treatment of porphyria cutanea tarda / Relação estrutura-função da enzima uroporfirinogênio descarbioxilase: implicações para a compreensão e tratamento da porfiria cutânea tardia / Reconocimiento a la trayectoria del Prof. Dr. Juan Miguel Castagnino†

Resumen La uroporfirinógeno descarboxilasa humana (UROD-h) es la quinta enzima del camino biosintético del hemo y su actividad deficiente, relacionada a mutaciones en su gen, se encuentra asociada a un subgrupo de porfirias. El objetivo de este trabajo fue estudiar la relación entre la dimerización de la enzima y su actividad enzimática y comprobar si la dimerización de UROD-h es imprescindible tanto para la primera etapa de la reacción (urogen→heptagen), como para la segunda etapa (heptagen→coprogen). Con ese objetivo, se expresó y purificó la UROD-h hasta homogeneidad, se analizó el comportamiento dímero-monómero bajo distintas condiciones que pudieran desplazar el equilibrio de dimerización y se evaluó la actividad enzimática en dichas condiciones. Los resultados obtenidos sugieren que la especie activa para la primera etapa de la reacción es el homodímero y que tanto el dímero como el monómero se comportan como especies activas para la segunda etapa de la reacción. Se propone que mutaciones clínicas como la Y311C, existentes en pacientes con porfiria cutánea tarda, podrían afectar la estabilidad del dímero y podrían ser el blanco para futuras terapias génicas.

Abstract Human uroporphyrinogen decarboxylase (UROD-h) is the fifth enzyme in the heme biosynthetic pathway and its deficient activity, related to mutations in its gene, is associated with a subset of porphyrias. The objective of this work was to study the relationship between the dimerisation of the enzyme and its enzymatic activity and to verify if the dimerisation of UROD-h is essential both for the first stage of the reaction (urogen→heptagen), and for the second stage (heptagen→ coprogen). With this objective, the UROD-h was expressed and purified to homogeneity, the dimer- monomer behaviour was analysed under different conditions, which could shift the dimerisation equilibrium, and the enzymatic activity was evaluated under these conditions. The results obtained suggest that the active species for the first stage of the reaction is the homodimer, and both the dimer and the monomer behaved as active species for the second stage of the reaction. It is proposed that clinical mutations such as Y311C, existing in porphyria cutanea tarda patients, could affect dimer stability and could be the target of future gene therapies.

Resumo A enzima uroporfirinogênio descarboxilase humana (UROD-h) é a quinta enzima da via biossintética do heme e sua atividade deficiente, relacionada com mutações em seu gene, está associada a um subgrupo de porfirias. O objetivo deste trabalho foi estudar a relação entre a dimerização da enzima e sua atividade enzimática e comprovar se a dimerização da UROD-h é imprescindível tanto para a primeira etapa da reação (urogênio→heptagênio), quanto para a segunda etapa (heptagênio→coprogênio). Com esse objetivo, a UROD-h foi expressa e purificada até a homogeneidade, o comportamento de dímero-monômero foi analisado sob diversas condições, que puderam deslocar o equilíbrio de dimerização, e a atividade enzimática foi avaliada em tais condições. Os resultados obtidos sugerem que a espécie ativa para a primeira etapa da reação é o homodímero, e tanto o dímero quanto o monômero se comportam como espécies ativas para a segunda etapa da reação. Propõe-se que mutações clínicas como Y311C, existentes em pacientes com porfiria cutânea tardia, poderiam afetar a estabilidade do dímero e poderiam ser o alvo de futuras terapias gênicas em porfiria cutânea tardia.

Relationship Between the Dose Administered, Target Tissue Dose, and Toxicity Level After Acute Oral Exposure to Bifenthrin and Tefluthrin in Young Adult Rats.

Most pyrethroid insecticides (PYRs) share a similar primary target site in mammals. However, the potency estimates of the lethal and sublethal effects of these compounds differ up to 103-fold. The aim of this study was to evaluate the relationship between the dose administered, the target tissue dose, and the effect of 2 highly toxic PYRs, tefluthrin (TEF; 0.1-9 mg/kg) and bifenthrin (BIF; 0.5-12 mg/kg), by using the oral route, a corn oil vehicle (1 ml/kg) and subcutaneous temperature (Tsc) monitoring assays in adult rats. The Tsc was determined at 30-min intervals for 5 h (TEF) or 4.5 h (BIF) after dosing. Rats were sacrificed at 6 h after dosing, and BIF and TEF concentrations were determined in blood (Bd), liver (Lv), and cerebellum (Cb) by using a GC-ECD system. The minimal effective dose of BIF (3 mg/kg) affecting Tsc was similar to that found in prior studies using other testing paradigms. Regarding TEF, a very steep relationship between the dose administered and toxicity was observed, with a near-threshold to low-effective range for Tsc at 0.1-6 mg/kg, and a near lethal syndrome at ≥ 7.5 mg/kg. At 6-7.5 mg/kg TEF, the Cb/Bd and Cb/Lv concentration ratios were both > 1. Conversely, for BIF, the Cb concentration was barely over the Bd concentration and the Cb/Lv concentration ratio remained < 1. Our results and previous findings call for more comprehensive consideration to establish the relevance of the distribution into target tissues and the tissue dosimetry for health risks through the exposure to PYRs in humans.

Relationship between exposure, body burden and target tissue concentration after oral administration of a low-dose mixture of pyrethroid insecticides in young adult rats.

Pyrethroids (PYRs) are synthetic insecticides increasingly used in agricultural and household pest control. Little is known on how the toxicity of highly effective bolus doses of single compounds compares to more realistic scenarios of low-level exposure to PYR mixtures. In this study, we examined a quaternary mixture of two noncyano (tefluthrin, TEF; bifenthrin, BIF) and two cyano (α-cypermethrin, α-CPM; deltamethrin, DTM) PYRs in young adult rats. These compounds are mostly composed of PYR isomers ranking top ten in acute lethality in rats. Concurrently, we administered near-threshold levels of the four PYRs dissolved in corn oil by oral route. Six hours later blood was collected and the liver and cerebellum were dissected out to determine PYR concentrations in these tissues using Gas Chromatography with Electron Capture Detector (GC-ECD). The mixture caused mild-to-moderate changes in non-locomotor behaviors and subcutaneous body temperature (up to +1.2-1.5 °C increase at 2-4 h after dosing, respectively, compared to pre-dosing records). The most toxic PYRs BIF and TEF reached higher concentrations in the cerebellum than the cyano-compounds α-CPM and DTM. In addition, PYR concentrations in the cerebellum were correlated to single compound proportions in the dosing solution and changes in body temperature. Our results suggest that aggregate exposures resulting in a target tissue burden of ∼10-1 nmoles PYR/g may be toxicologically relevant, expanding the evidence on exposure-dose-effect relationships for PYRs, and serving to design convenient pharmacokinetic models for environmentally relevant exposures to PYR mixtures.