METHIONINE FLUX TO TRANSSULFURATION IS ENHANCED IN THE LONG LIVING AMES DWARF MOUSE

Authors: Uthus, Eric; BROWN-BORG, HOLLY - UNIV OF NORTH DAKOTA

Submitted to: Mechanisms of Aging and Development
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2006
Publication Date: 3/6/2006
Citation: Uthus, E.O., Brown-Borg, H.M. 2006. Methionine flux to transsulfuration is enhanced in the long living Ames dwarf mouse. Mechanisms of Aging and Development. 127:444-450.

Interpretive Summary: The Ames dwarf mouse lives up to 64% longer than their normal siblings. The reason for their increased lifespan is not known. These mice, however, are deficient in growth hormone, prolactin, and thyroid-stimulating hormone. These hormones can affect various processes including the metabolism of certain amino acids (building blocks of protein) including methionine. Also, we previously reported that the dwarf mice exhibit enzyme activities and levels that combat oxidative stress more efficiently than normal mice. Thus, we performed an experiment that compared various parameters of methionine metabolism between 17-18 month old male dwarf and wild type (normal) mice. The speed at which methionine is broken down (metabolized by enzymes) was dramatically increased in the Ames dwarf mouse compared to normal mice. This would indicate that the enzymes involved in the metabolism of methionoine are elevated in the Ames mouse. This was confirmed by looking at the amount of mRNA (genetic material), which dictates how much of an enzyme the mouse will produce. In the case of the Ames dwarf, the amount of mRNA was elevated by 100-300% for many of the enzymes used to metabolize methionine. These data indicate that dwarf mice, compared to wild type mice, have a markedly accelerated metabolism of methionine. Because of methionine or metabolites of methionine are involved in anti-oxidative processes, the enhanced methionine metabolism may partially explain earlier reports indicating less oxidative damage to proteins in dwarf mice. Taken together, the data suggest that methionine metabolism may play a role in oxidative defense in the dwarf mouse in extending its extended lifespan.

Technical Abstract: Long-lived Ames dwarf mice lack growth hormone, prolactin, and thyroid stimulating hormone. Additionally the dwarf mice have enzyme activities and levels that combat oxidative stress more efficiently than those of normal mice. We have shown that methionine metabolism in Ames mice is markedly different than in their wild type littermates. In our previous work we hypothesized that the flux of methionine to the transsulfuration pathway is enhanced in the dwarf mice. The current study was designed to determine whether the flux of methionine to the transsulfuration pathway is increased. We did this by injecting either L-[methyl-3H]-methionine or L-[35S]-methionine into dwarf or normal mice and then determined retained label (in form of S-adenosylmethionine) 45 min later. The amount of retained hepatic 3H and 35S label was significantly reduced in the dwarf mice; at 45 min the specific radioactivity of SAM (pCi /nmol SAM) was 56% lower (p<0.05) for 3H-label and 64% lower (p<0.005) for 35S-label in dwarf than wild type mice. Retention of 35S was significantly lower in the brain (37%, p<0.04) and kidney (47%, p<0.02) of the dwarf compared to wild type mice; there was no statistical difference in retained 3H-label in either brain or kidney. This suggests that both the methyl-moiety and the carbon chain of methionine are lost much faster in the dwarf compared to the wild type mouse, implying that both transmethylation in the liver and transsulfuration in the liver, brain and kidney are increased in the dwarf mice. As further support, we determined by real-time RT PCR the expression of methionine metabolism genes in livers of mice. Compared to wild type, the Ames dwarf had increased expression of methionine adenosyltransferase 1a (2.3-fold, p=0.013), glycine N-methyltransferase (3.8-fold, p=0.023), betaine homocysteine methyltransferase (5.5-fold, p=0.0006), S-adenosylhomocysteine hydrolase (3.8-fold, p=0.0005), and cystathionase (2.6-fold; tended to be increased, p=0.055). Methionine synthase expression was significantly decreased in dwarf compared to wild type (0.48-fold, p=0.023). These results confirm that the flux of methionine to transsulfuration is enhanced in the Ames dwarf. This, along with data from previous studies support the hypothesis that altered methionine metabolism plays a significant role in the oxidative defense of the dwarf mouse and that the mechanism for the enhanced oxidative defense may be through altered GSH metabolism as a result of the distinctive methionine metabolism.