Protein Misfolding and Cellular Stress: An Overview - Protein Misfolding and Cellular Stress in Disease and Aging - page 16

6. Reactive
Carbonyl
Compounds:
Creation and
Protein
Modification

As mentioned earlier, the proteolytic systems in the cytosol (the
proteasome) as well as that in the mitochondria (the Lon pro-
tease) are able to degrade modified proteins ( 21, 47 ). The pres-
ence of these systems in young and healthy cells signifies a need
to eliminate modified proteins produced continuously as byprod-
ucts from “normal” and stress-induced ROS/RNS production.
In this connection, it is noteworthy that a protein mass spectro-
metric analysis of carbonylated proteins from a mitochondria
enriched fraction from the hind limb muscle of young (18
month old) female adult rats revealed 243 carbonylated proteins
of which 94 were localized to the mitochondria ( 77 ) (see below).
To set this in perspective, it is of special interest in the present
context that the activity of both the proteasome and the Lon
protease decline with age ( 21, 47 ), indicating that accumulation
of numerous modified proteins are important contributors to
age-related diseases as well as to the general ageing. This view is
compatible with the modern theories of ageing ( 78, 79 ) (see also
Chap. 7).
Carbonylation of proteins may arise predominantly from two
sources, lipid peroxidation and oxidative break-down of glycated
proteins ( 18, 80, 81 ). Lipid peroxidation occurs when unsatu-
rated fatty acid components of lipids are attacked by nucleophilic
ROS and RNS molecules. The primary products are subsequently
metabolized through metabolic and detoxification pathways to
small molecular compounds, of which the best known are alde-
hydes, for example, malondialdehyde, hexanal, acrolein, glyoxal,
crotonaldehyde, 2-nonenal, 4-oxo-2-nonenal, and 4-hydroxy-2-
nonenal. Glycated proteins on the other hand are produced from
sugars containing reducing aldehyde groups, such as glucose,
which react with free amino groups (arginine and lysine) in pro-
teins to form so-called Schiff bases. Subsequent rearrangements
of these bases give rise to advanced glycation endproducts (AGE).
If not accumulated or excreted, these AGEs may be oxidatively
metabolized to small molecules, such as the dicarbonyls methyl-
glyoxal, glyoxal and 3-deoxyglucosone, as well as the aldehydes
diacetyl, acetol, pyruvaldehyde and acrolein ( 18 ). This probably
unfinished list of carbonyl compounds, which may all react non-
enzymatically with proteins, DNA as well as other small mole-
cules, such as ketone bodies (Poulsen T and Johannsen M 2009,
unpublished), emphasizes the size of the problem for the cell to
cope with.
As mentioned above, a crudely mitochondrial enriched frac-
tion of young rat muscle contained 243 identified carbonylated
proteins, of which 94 could be assigned mitochondrial ( 77 ). The
study was methodological in its nature and designed to evaluate