Para-aminophenol and structurally related compounds as intermediates in lipofuscin formation and in renal and other tissue toxicities.

P-aminophenol is considered a minor nephrotoxic metabolite of phenacetin and acetaminophen (paracetamol) in man. Our experiments show that p-aminophenol readily undergoes oxidative polymerization during incubation in human blood or plasma, to form melanin, as a component of soluble lipofuscin. Haemolysis accompanies this process in whole blood. Unmetabolized phenacetin and acetaminophen do not form soluble lipofuscins. Long-term excessive use of phenacetin or acetaminophen has been associated with chronic renal disease, haemolytic anaemia, and increased solid lipofuscin deposition in tissues. Excessive use of phenacetin has also been associated with cancer of renal pelvis and bladder. It appears to us that p-aminophenol and other o- and p-aminophenol metabolites of these drugs are intermediates not only in the etiology of chronic renal disease, but in the other developments as well. P-aminophenol and other ex(end)ogenous aminohydroxyphenyl, aminopolyhydroxyphenyl, polyhydroxyphenyl and polyaminophenyl compounds with these groups in ortho and para positions (such as 3-hydroxyanthranilic acid, 6-aminodopamine, dopamine, p-phenylenediamine, etc.) can undergo autoxidations and metal-catalyzed and enzymatic oxidations in man to produce toxic (semi)quinones(imines), (semi)quinonediimines and reactive oxygen species. After depletion of antioxidants these very reactive (semi)quinones(imines) and (semi)quinonediimine intermediates, many of which are precursors of plasma soluble lipofuscins and melanoproteins, react with essential proteins, DNA, other macromolecules and can cause or contribute to renal and other tissue toxicity, haemolytic anaemia, neoplasia, and granular lipofuscin formation. The reactive oxygen species can also deplete antioxidants, damage essential proteins, DNA, and other macromolecules, and thereby injure cells and extracellular matrix.