Aging differentially affects the uptake of methionine by select rat tissues

Experimental and epidemiological studies indicate that significant reductions in dietary methionine intake profoundly impact both health and life spans. Methionine levels also decrease in the blood of mammals as they age. Here, we report that methionine uptake by four of the major methionine-consuming organs, the liver, kidneys, heart, and brain, is increased in old rats as compared to young adult rats, as measured by [¹¹C]methionine uptake by these organs using positron emission tomography. This increased uptake was sustained for at least 30 min and resulted in 1.6- to 1.9-fold increases in methionine uptake by the major methionine-consuming organs in old rats, suggesting an age-associated acceleration of methionine metabolism and its associated biochemical pathways. By contrast, the uptake of [¹⁸F]fluorodeoxyglucose by the liver, kidneys, heart, and brain of older rats was reduced relative to that by their younger adult counterparts, indicating a decline in glucose metabolic activity with age. Remarkably, the uptake of 2-[¹⁸F]fluoropropionic acid by these organs remained essentially unchanged between young and aged adult rats, suggesting the presence of a stable biochemical equilibrium in propionate metabolism across the aging spectrum. These findings underscore the utility of positron emission tomography in revealing significant age-related alterations in organ-specific biochemical processes.