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possibly methionine overrepresentation at the surface of proteins. In different
species, which own similar metabolic potentials, they seem to be inversely
associated with rates of reactive oxygen species (ROS) generation (2). The
so-called Great Oxidation Event is thought to have resulted from the inven-
tion of the photosystem II by cyanobacteria between 2.7 and 1.9 billion years
ago and raised the atmospheric level of oxygen to the present level from less
than 10 −5 times that value (4). Since then, some organisms have remained
anaerobes and are typically very sensitive to molecular oxygen, whereas
most have developed aerobic respiration. Because oxygen diffuses through
the membranes to enter the cell, membrane, proteins are expected to show
signs of adaptation to high oxygen concentrations. Transmembrane helices
of proteins of eukaryotes have longer oxygen-rich outer domains than those
of prokaryotes. This led to the suggestion that the rise of molecular oxygen
concentration allowed the expansion of these domains and led to the devel-
opment of communication-related transmembrane proteins among eukary-
otes (5).
It is somewhat difficult to study the aging process in humans due to the
difficulty to get human material, the large variety of factors influencing a
human life, including diet, environment, disease, and living conditions. There-
fore, it is difficult to find basic mechanisms of the aging process in humans and
reveal the underlying processes. This includes the reasons for the accumulation
of oxidized proteins with age. Consequently, a set of models have been devel-
oped and might also be used to highlight aspects of human aging. This includes
cellular models and model organisms for the measurement of protein oxida-
tion kinetics, protein accumulation, and protein turnover. Some of the pre-
ferred models are mammalian or human cells such as fibroblasts or endothelial
cells, or organisms with a short life span, such as the Drosophila elegans
or Caenorhabditis elegans . Numerous studies have also been performed in
rodents and some in monkeys, since the results of mammals, including nonhu-
man primates, are expected to be better transferred to humans compared to
nonmammalian models (6).
Several protein oxidation products may show different characteristics
according to various species. Hence it is important to have results from differ-
ent species. For example, carbonyl levels are higher in crawlers (low life expec-
tancy) than fliers in a cohort of houseflies of the same chronological age (7).
In the case of chronological aging of Escherichia coli cells, the cells displaying
low carbonyl levels remain reproductively competent, whereas cells with a
high carbonyl load become genetically dead (8). Interestingly, the plant Ara-
bidopsis thaliana appears to have evolved mechanisms that allow carbonyl-
ation to drop abruptly prior to the vegetative-to-reproductive transition (9),
whereas in mice, carbonylated proteins are eliminated upon early embryonic
development of the blastocysts (10). The asymmetrically dividing yeast Sac-
charomyces cerevisiae has evolved a Sir2p-dependent system that segregates
carbonylated proteins during mitotic cytokinesis, ensuring low levels of oxida-
tive damage in the progeny (11).
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