Physico-chemical aspects in silicosis

Abstract

Silicosis is a pulmonary disease which afflicts persons who inhale over long periods of time freshly-ground silica-containig dust. Its mechanism of action at the molecular level is not yet understood. The majority of crystalline SiO 2 forms are pathogenic whereas the amorphous silicas are largely inactive. The primary cause of silicosis has thus to be sought in the structural and surface characteristics of the SiO 2 particles. Recent biological work envisages in the engulfment of the SiO 2 particles into a macrophage the first step ending up with the formation of the silicotic nodule in the lung [1–3]. Intermediate stages of the process are illustrated in Fig. 1. Any silica particle ▪ exhibits a membranolytic action on the phagolysosome, probably related to the surface hydroxyls configuration, with consequent release of lytic enzymes into the cytoplasm, death of the macrophage and release of the free SiO 2 particle in the lung tissue where it can be phagocytosed by another macrophage. This does not imply any fibrotic action per se. If the process takes place with fresh ground, crystalline SiO 2 , namely quartz, tridymite and cristobalite, within the phagolysosome a fibrogenic factor is also formed, which, when released in the lung tissue, stimulates fibroblasts to an abnormal production of collagen, and hence the formation of the silicotic nodule. Membranolysis can be reduced or blocked by chemical modification of the surface, however the intimate cause of silicosis relies on the catalytic role of crystalline SiO 2 in the production of the ‘factor’. Virtually any difference in surface properties between amorphous and crystalline silicas may account for their different biological activities. We have investigated, so far, the formation of free radicals at the crushed surface (E.S.R.), the heat of interaction with water molecules (adsorption microcalorimetry) and the kinetics and energy of interaction with some aminoacids (immersion calorimetry) on amorphous and crystalline silicas of comparable size [4]. Free radicals are produced by mechanical grinding of quartz, and readily react with various atmospheric components yielding paramagnetic species such as SiO • 2 and SiCO • 2, possible intermediates in the formation of the fibrogenic factor. Water vapour reacts with silicas in various ways depending on the dehydration degree of the surface. The heat of adsorption on micronized quartz (∼ 5 M 2 g −1), low and high surface area amorphous silicas (porasil ∼ 16 m 2 g −1), (aerosil ∼ 380 m 2 g −1 all out-gassed in vacuo below 423 K (in order to prevent elimination of silanols), are reported in Fig. 2 as a function of water uptake. Micronized quartz exhibits at low coverages an interaction energy (∼ 210 kJ mol −1) which is much higher than corresponding one on the two amorphous silicas (∼ 125 kJ mol −1). ▪ Crystalline structure rather than particle size thus dictates the binding energy of surface sites. Aminoacids are adsorbed at the quartz surface [5] but when the process occurs from aqueous solution their interaction energy is competitive with water itself. In the case of proline, however, a specific interaction with quartz was observed: in that case the heat released upon contact with quartz was 30 times higher than on amorphous silica and the interaction lasted many hours indicating an activated process, possibly the oxidation to hydroxyproline specifically catalyzed by the quartz surface. All these findings may be regarded as a first step in the investigation of the particular reactivity of crystalline SiO 2 within the cells.