by J. Travis
People don't normally throw away their silverware after a single meal. It's usually cheaper to wash and reuse the utensils than to buy new ones.
Similarly, cells may sometimes do better to repair partially damaged proteins than waste a fair amount of energy synthesizing replacements. In test-tube experiments, scientists had identified a few repair mechanisms that cells may employ, but they had obtained little information on whether animals actually depend on such molecular restoration.
Dramatic proof that they do is now at hand. Investigators who created mice bereft of an enzyme implicated in protein repair have discovered that the seemingly healthy rodents suffer fatal seizures within a month or two of birth.
The enzyme's absence "has rather dire consequences. It's an exciting development and emphasizes the importance of this repair pathway," says Earl R. Stadtman, a protein repair investigator at the National Heart, Lung, and Blood Institute in Bethesda, Md.
Compared to the crush of scientists examining how cells fix DNA damage, few researchers have focused on the natural deterioration of proteins as cells age. In recent years, however, some scientists have championed a theory of aging that blames the gradual accumulation of damaged proteins, many of them generated by interactions with highly reactive, oxygen-bearing molecules known as free radicals (SN: 5/18/96, p. 311).
Other forms of protein damage also occur naturally, including spontaneous degradation of amino acids such as asparagine and aspartic acid. Such damage involves "a shuffling around of atoms within the molecules. It can grossly affect the whole structure of a protein," notes Edward Kim of the University of California, San Francisco.
Scientists had found an enzyme called both L-isoapartyl (D-aspartate) O-methyltransferase and PCMT-1 that in test tubes could heal proteins with this type of amino acid injury. However, they didn't know whether cells use the enzyme in this way or simply junk damaged proteins and make new ones. Kim and his colleagues genetically engineered mice to lack PCMT-1. Though significantly smaller than normal, the rodents seemed healthy. After several weeks, however, the situation changed. When researchers left the lab for the evening, or even a few hours, they would often find a dead mouse when they returned.
Autopsies revealed no obvious abnormalities, so the investigators began videotaping the mice around the clock. They discovered that the animals suffered fatal seizures. Their muscles would begin to twitch, and after several minutes they would run around or jump for 15 to 20 seconds. Finally, they would fall on their sides and stop breathing.
Since the seizures suggested abnormal brain activity and PCMT-1 is most abundant in the brain, the scientists examined cells there for damaged proteins. They found plenty. About 6 percent of the proteins inside the cells were damaged, they estimate in the June 10 Proceedings of the National Academy of Sciences. Normally, less than 1 percent are faulty, says Kim.
How the absence of PCMT-1 generates seizures remains unclear, though Kim and his colleagues speculate that the accumulation of damaged proteins may interfere with the metabolism of glutamate, a crucial neurotransmitter. The question of PCMT-1's relevance to aging remains open. "The premature death of the mice precludes a test of [the enzyme's] role in aging," notes George M. Martin, a gerontologist at the University of Washington in Seattle.
Kim and his colleagues agree, and they now plan to test the aging theory by generating mice that make PCMT-1 in the brain but nowhere else. That should prevent the seizures and may allow the mice to live long enough for other organs to exhibit difficulty in dealing with damaged proteins, says Kim.
Kim, E., et al. 1997. Deficiency of a protein repair enzyme results in the accumulation of altered proteins, retardation of growth, and fatal seizures in mice. Proceedings of the National Academy of Sciences 94(June 10):6132.
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