What is the rate-limiting step towards aging? Chemical reaction kinetics might reconcile contradictory observations in experimental aging research

Abstract

Modern geroscience is divided as regards the validity of the free radical theory of aging. Thermodynamic arguments and observations from comparative zoology support it, whereas results from experimental manipulations in representative animal species sometimes strongly contradict it. From a comparison of the multi-step aging process with a linear metabolic pathway (glycolysis), we here argue that the identification of the rate-limiting kinetic steps of the aging cascade is essential to understand the overall flux through the cascade, i.e., the rate of aging. Examining free radical reactions as a case in point, these reactions usually occur as chain reactions with three kinetically independent steps: initiation, propagation, and termination, each of which can be rate-limiting. Revisiting the major arguments in favor and against a role of free radicals in aging, we find that the majority of arguments in favor point to radical propagation as relevant and rate-limiting, whereas almost all arguments in disfavor are based on experimental manipulations of radical initiation or radical termination which turned out to be ineffective. We conclude that the overall lack of efficacy of antioxidant supplementation (which fosters termination) and antioxidant enzyme overexpression (which inhibits initiation) in longevity studies is attributable to the fact that initiation and termination are not the rate-limiting steps of the aging cascade. The biological and evolutionary plausibility of this interpretation is discussed. In summary, radical propagation is predicted to be rate-limiting for aging and should be explored in more detail.

Keywords: Free radical theory of aging; Metabolic flux; Radical initiation; Radical propagation; Radical termination; Rate-limiting step.

Fig. 1

Glycolysis and aging, two multi-step kinetic processes. Glycolysis is a linear metabolic pathway for the degradation of glucose to pyruvate. The flux through this pathway is governed by a few rate-limiting, flux-controlling steps, whereas other steps, which are similarly indispensable for the overall pathway, are not rate-limiting. In the present representation, the thickness of the arrows (1)–(10) reflects the maximal reaction rates v_max of each individual step of glycolysis in the working rat heart, adopting data from Kashiwaya et al. (1994). Glycolysis in the rat heart is predominantly controlled by the slow hexokinase (1) and enolase (9) reactions, whereas the 3-phosphoglycerate kinase reaction (7) is never rate-limiting. As argued in the text, the biological aging process shares relevant aspects with linear metabolic pathways such as glycolysis, including the possible existence of rate-limiting steps. However, the reaction rates of the individual steps in the modeled aging cascade are unknown, as is the identity of the rate-limiting steps of the cascade. Spontaneous chemical transformations like oxidation, isomerization, or hydrolysis are likely involved in the aging cascade at some point, but it is unknown so far whether they are rate-limiting. In the depicted example, the enzyme catalyzing the transition from state X to state Y is not rate-limiting; hence, its essential role for aging can neither be determined by overexpression nor by knockdown, unless the knockdown is approaching 100% and no isozymes exist. GAP glyceraldehyde-3-phosphate, DHAP dihydroxyacetone phosphate

Fig. 2

Generalized kinetics of free radical chain reactions. Free radical chain reactions are characterized by their three essential steps: initiation, propagation, and termination. These reactions exhibit different rate constants k and reaction rates r as linked in their rate laws. During initiation, initiator radicals I● are formed which attack substrate molecules S. In general, the subsequent attack on S is much faster than I● formation, resulting in the depicted, simplified rate law for initiation. Propagation is characterized by the formation of the product P out of S●. Termination involves the recombination of two radicals or the disproportionation of two radicals. The latter variant is not shown, but intended to be included in the overall rate law of termination with termination constant k_T. Chain-transfer agents offering alternative reaction pathways for propagation or chain-breaking antioxidants offering alternative reaction pathways for termination were omitted for clarity. These agents are usually catalysts and do not change the shape of the corresponding rate laws but increase the apparent rate constants of propagation and termination. In the combined rate law for the overall process, the substrate concentration [S] and the propagation constant k_P enter linearly, whereas the initiator concentration [I₂], the initiation constant k_I, and the reciprocal termination constant 1/k_T only enter as square roots