Porphyrin metabolism in lead and mercury treated bajra (Pennisetum typhoideum) seedlings

Lead and mercury inhibited porphyrin biosynthesis significantly in the germinating seeds of bajra (Pennisetum typhoideum). Both 5-aminolevulinic acid dehydratase and porphobilinogen deaminase activities were inhibited by these metals. A comparative study of the inhibition of these two enzymes under invivo andin vitro conditions showed that 5-aminolevulinic acid dehydratase is the major site of action of heavy metals in porphyrin biosynthesis. Further, over-all production of porphyrinsviz., protoporphyrin-IX, Mg-protoporphyrin (ester) and protochlorophyllide was repressed by lead and mercury in both light and dark grown seedlings. Similarly, chlorophylla and chlorophyllb and total chlorophyll contents in dark-grown seedlings were also significantly decreased, suggesting the impairment of chlorophyll biosynthesis by lead and mercury in germinating seedlings.