Saccharomyces cerevisiae Mutants Defective in Heme Biosynthesis as a Tool for Studying the Mechanism of Phototoxicity of Porphyrins

Abstract

Mutants of Saccharomyces cerevisiae accumulating uroporphyrin (UP) or protoporphyrin (PP) were used as a model for the in vivo phototoxic effect of porphyrins observed in the human skin photosensitivity associated with porphyrias (porphyria cutanea tarda and erythropoietic protoporphyria). We have found that UP is localized in vacuoles and PP is present in all compartments except vacuoles in yeast cells. Endogenous PP is much more effective as a photosensitizer of yeast cells than UP. Protoporphyrin action is strictly dependent on the presence of oxygen. In contrast, UP displays a phototoxic effect even if oxygen is not present in the suspension, implicating a free radical mechanism that operates in anaerobiosis upon photosensitization by UP. Catalase or superoxide dismutase deficiency affects photosensitization by UP. A possible mechanism of UP photosensitizing activity is discussed.

Ferrochelatase is a conserved downstream target of the blue light-sensing White collar complex in fungi

“…This was the primary light-induced gene (Table 3), and warranted further study because ferrochelatase mutations cause photosensitivity in all organisms in which mutants have been examined. These organisms include the bacterium Escherichia coli , in which ferrochelatase mutants also become light-sensitive and negatively phototactic (Miyamoto et al , 1991; Yang et al , 1995), and the non-photosensing fungus S. cerevisiae (Abbas & Labbe-Bois, 1993; Zoladek et al , 1996). Mutation of ferrochelatase is one basis of porphyria, a class of genetic disorders in humans, in which ferrochelatase impairment leads to severe sensitivity to sunlight and subsequent skin damage if exposed (Cox, 1997; Kauppinen, 2005).…”

The Escherichia coli Protein YfeX Functions as a Porphyrinogen Oxidase, Not a Heme Dechelatase

“…This last process has not been observed in bacteria and, if it does exist, it would be inadequate to eliminate intracellular protoporphyrin that accumulates when bacterial cultures are supplemented with ALA, the first committed intermediate in the biosynthesis of heme. In S. cerevisiae lacking the gene for ferrochelatase ( hem15 ), protoporphyrin accumulates in the cytosol, and when the cells are illuminated, the porphyrin has been shown to be cytotoxic (29), supporting the observation that accumulation of protoporphyrin is undesirable. In mice that have a homozygous deletion of IRP2, the primary iron-regulatory protein in the erythron, lack of iron leads to accumulation of protoporphyrin (30, 31), further supporting the conclusion that in the absence of iron the demand for heme leads to protoporphyrin accumulation.…”

Edible mycelium bioengineered for enhanced nutritional value and sensory appeal using a modular synthetic biology toolkit

“…15). Importantly, high levels of free heme and the porphyrin intermediates can be toxic to the cell, causing oxidative damage and hampering growth 77,83 . To address this potential challenge, we expressed two copies of Soy Leghemoglobin, the FDAapproved protein used in IMPOSSIBLE meat 72 , as a potential heme sink, using both the SES and the significantly weaker pTEF1 promoter.…”