Because our measurements have shown no difference in either the extent or pattern of HNE-modified proteins, one possibility is that the cellular detoxification systems directed at HNE are adequate regardless of the disease stage. Previous studies have shown that the detoxifying enzymes are upregulated when the cells are exposed to oxidative stress.
35 36 37 38 Therefore, the cellular content of these enzymes provide an indication of the cells’ response to oxidative stress. We evaluated the content of several proteins involved in detoxifying or removing HNE from the cell as an indirect indicator of cellular levels of HNE. We measured the content of three enzymes that directly detoxify HNE in the cell: aldehyde dehydrogenase (ALDH), aldose reductase (AR), and glutathione-
S-transferase (GST). These enzymes detoxify HNE via oxidation to 4-hydroxy-2-nonenoic acid (HNA),
45 reduction to 1,4-dihydroxy-2-nonene (DHN),
46 or conjugation with glutathione (GSH).
47 Two of the three proteins directly involved in detoxification show a modest increase in expression. Our results show ALDH was significantly increased in content (
P = 0.02) and GSTpi showed a trend toward a significant increase (
P = 0.07), although AR exhibited no change in content (
P = 0.85;
Fig. 5 ). In addition, glutamyl cysteine synthetase (GCS), the rate-limiting enzyme in the formation of GSH, showed no significant change (
P = 0.53) in content when considering all four MGS stages. However, a linear increase (
P = 0.04) of GCS content was observed through MGS3.