Abstract
In barley (Hordeum vulgare L.) root cells, activity for oxidizing protoporphyrinogen to protoporphyrin (protoporphyrinogen oxidase), a step in chlorophyll and heme synthesis, was found both in the crude mitochondrial fraction and in a plasma membrane enriched fraction separated by a sucrose gradient technique utilized for preparing plasma membranes. The specific activity (expressed as nanomoles of protoporphyrin formed per hour per milligram protein) in the mitochondrial fraction was 8 and in the plasma membrane enriched fraction was 4 to 6. The plasma membrane enriched fraction exhibited minimal cytochrome oxidase activity and no carotenoid content, indicating little contamination with mitochondrial or plastid membranes. Etioplasts from etiolated barley leaves exhibited a protoporphyrinogen oxidase specific activity of 7 to 12. Protoporphyrinogen oxidase activity in the barley root mitochondrial fraction and etioplast extracts was more than 90% inhibited by assay in the presence of the diphenyl ether herbicide acifluorfen methyl, but the activity in the plasma membrane enriched fraction exhibited much less inhibition by this herbicide (12 to 38% inhibition) under the same assay conditions. Acifluorfen-methyl inhibition of the organellar (mitochondrial or plastid) enzyme was maximal upon preincubation of the enzyme with 4 mm dithiothreitol, although a lesser degree of inhibition was noted if the organellar enzyme was preincubated in the presence of other reductants such as glutathione or ascorbate. Acifluorfen-methyl caused only 20% inhibition if the enzyme was preincubated in buffer without reductants. Incubation of barley etioplast extracts with the earlier tetrapyrrole precursor coproporphyrinogen and acifluorfen-methyl resulted in the accumulation of protoporphyrinogen, which could be converted to protoporphyrin even in the presence of the herbicide by the addition of the plasma membrane enriched fraction from barley roots. These findings have implications for the toxicity of diphenyl ether herbicides, whose light induced tissue damage is apparently caused by accumulation of the photoreactive porphyrin intermediate, protoporphyrin, when the organellar protoporphyrinogen oxidase enzyme is inhibited by herbicides. Our results suggest that the protoporphyrinogen that accumulates as a result of herbicide inhibition of the organellar enzyme can be oxidized to protoporphyrin by a protoporphyrinogen oxidizing activity that is located at sites such as the plasma membrane, which is much less sensitive to inhibition by diphenylether herbicides.