Age-Dependent Denaturation of Enzymes in the Human Lens: A Paradigm for Organismic Aging?

Abstract

Little is known about the rate of denaturation of proteins within the human body. To monitor this decline, human eye lenses were dissected into discrete regions that were formed at different stages of life and assayed for activity of lactate dehydrogenase (LDH) and a particularly stable enzyme, glutathione reductase (GR). Activity was highest for both enzymes in the most recently synthesized outer part of the lens, decreased further into the lens, and, for LDH, was barely detectable in nuclear regions that consist of proteins that were synthesized in utero. For LDH, 95% of total lens activity was found in the outer half of the adult lens at all ages. Activity was unchanged in the outermost part of the lens as a function of age, suggesting that the ability of humans to synthesize the two enzymes is not impaired, even up to the tenth decade. After age of 40, LDH activity declined steadily in the interior of the lens at the rate of 8.3% per decade. GR activity diminished more slowly, and western blotting indicated that both denaturation of the enzyme and truncation were responsible. These data support the view that few, if any, metabolic pathways remain in the center of older lenses. Exposure of the enzymes to physiological pH and temperature over a period of decades is presumably sufficient to cause denaturation. The center of older human lenses is a unique environment in which the accumulation of untoward posttranslational modifications to proteins can be studied in the absence of significant enzymatic amelioration.

Racemization of Two Proteins over Our Lifespan: Deamidation of Asparagine 76 in γS Crystallin Is Greater in Cataract than in Normal Lenses across the Age Range

“…It is unlikely that any methyltransferase activity is available in the center of adult lenses, since enzymes appear to be denatured by decades of exposure to body temperature. 57,58 The regions of both gamma S and aA crystallin that were examined in this study are located in flexible regions of the native proteins. [59][60][61] Such parts appear to be more susceptible to the post-translational modifications of the type characterized here than Asp/Asn residues located within structured portions.…”

Instability of the cellular lipidome with age

“…The pronounced changes with age are unlikely to be due to the activity of lens enzymes. Although phospholipase activity has been observed in the whole lens, even at 60 years of age (Kamei 1996), several separate studies indicate that metabolic pathways are absent from the centre of adult human lenses (Dovrat and Gershon 1981;Dovrat et al 1984;Scharf et al 1987;Zhu et al 2010b). Fiber cells in the lens nucleus lack organelles and, since there is no protein turnover (Lynnerup et al 2008), it is likely that the enzymes which were active in the centre of young lenses have been denatured due to decades of exposure to body temperature.…”

“…The lens grows continuously throughout life with newly differentiated fiber cells stacked onto the outside of a pre-existing core. Mature fiber cells in the lens centre have no organelles (Bantseev et al 1999) and lack significant metabolic activity (Zhu et al 2010b). Since proteins are present for our lifespan (Lynnerup et al 2008) and there is also evidence to suggest a lack of lipid turnover (Borchman et al 1999), the centre of the human lens can be used as a model system to examine the effects of ageing on the longterm stability of lipids within a biological environment.…”

Spontaneous cleavage of proteins at serine and threonine is facilitated by zinc

“…One example is a sterol‐regulated protease present in the endoplasmic reticulum (Duncan et al ., 1997). In the adult human lens, no active proteases remain in the centre (Zhu et al ., 2010) due to the particular growth pattern of the lens, so this is an excellent tissue to examine protein cleavage processes that occur spontaneously. In this tissue, several cleavage events on the N‐terminal side of Ser and Thr have been documented (Lampi et al ., 1998; Schey et al ., 2000; Santhoshkumar et al ., 2008; Su et al ., 2012).…”