A natural factor catalyzing reduction of methaemoglobin by isolated chloroplasts

Abstract

The crude or unwashed preparations of chloroplasts from a variety of plants will reduce muscle methaemoglobin in the light, but not in the dark. The chloroplasts after washing do not reduce the methaemoglobin unless a soluble factor present in the crude chloroplast suspension is restored. The factor is present in acetone leaf extract, and the activity is equivalent to that in a corresponding amount of the crude chloroplast suspension. In the two plants examined evidence was obtained for the relative absence of the factor in the parts of the plant free from chlorophyll. The factor, provisionally called ‘the methaemoglobin reducing factor’, is thermolabile, not dialyzable and precipitated with nearly saturated ammonium sulphate. It is stable between pH 5 and 9 at 20° C. These properties could be associated with those of a protein. The rate of hydrogen transfer in the reduction of the methaemoglobin by the factor plus chloroplasts is, in terms of the chlorophyll content, of the same order as the direct oxygen-producing reaction with the artificial hydrogen accepting reagents. For chloroplasts of Pisum sativum an optimum at pH 8 was found. The activity of the system is about halved by the presence of M/5 concentration of a variety of neutral salts. It is concluded that the factor is reduced directly by the illuminated chloroplasts, that oxygen is produced in the process, and that the factor acts in the sense of a catalyst for the reduction of methaemoglobin in the illuminated system.