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N-Methyl Mesoporphyrin IX Inhibits Phycocyanin, but Not Chlorophyll Synthesis in Cyanidium caldarium¹

Division of Biology and Medicine, Brown University, Providence, Rhode Island 02912

The ability of N-methyl mesoporphyrin IX (NMMP) to block heme synthesis by specifically inhibiting enzymic iron insertion into protoporphyrin IX was exploited to test whether heme is a precursor of the bilin chromophore of phycocyanin (PC). A strain of the unicellular rhodophyte Cyanidium caldarium which forms normal amounts of both chlorophyll (Chl) and PC in the dark was employed to avoid phototoxic effects of exogenous porphyrins. Relative Chl and PC content were assayed spectrophotometrically on whole cell suspensions.

When cells were grown in the dark on a glucose-based heterotrophic medium at 42°C, neither division rate nor Chl synthesis was affected by NMMP up to 3.0 micromolar and for as long as 72 hours. NMMP had a dose-dependent inhibitory effect on PC synthesis. PC to Chl absorbance ratios, relative to control cell values, were 100%, 89%, 86%, and 50% in cells grown for 48 hours with 0.3, 1.0, 3.0, and 10.0 micromolar NMMP, respectively. NMMP also caused the accumulation of intracellular protoporphyrin.

The ability of NMMP to cause intracellular accumulation of protoporphyrin and to block PC synthesis specifically while allowing normal Chl formation is consistent with its action as a specific inhibitor of enzymic iron chelation, and supports the role of heme as a precursor to the phycobilins.

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				S.-H. Wu, M. T. McDowell, and J. C. Lagarias Phycocyanobilin Is the Natural Precursor of the Phytochrome Chromophore in the Green Alga Mesotaenium caldariorum J. Biol. Chem., October 10, 1997; 272(41): 25700 - 25705. [Abstract] [Full Text] [PDF]