Age-related Accumulation of 1-methyl-1,2,3,4-tetrahydro-β carboline-3-carboxylic acid in Human Lens

Abstract

A fluorescent β-carboline, 1-methyl-1,2,3,4-tetrahydro-β -carboline-3-carboxylic acid (MTCA) was identified in human normal lenses and cataractous lenses. The mean total amounts of MTCA in normal lenses ( n=7), diabetic cataractous lenses ( n=15), and senile cataractous lenses ( n=15) were 0.64±0.76ng, 1.79±0.94ng, and 3.66±1.04ng, respectively. The MTCA levels in human lenses increased linearly with the age of the lenses, and there was a significant correlation between the total amount of MTCA in human lenses and the age of the lens. In all types of lenses, the concentrations of MTCA in the insoluble-protein fraction [pg(mg protein) -1] were significantly higher than those in the water-soluble protein fraction. These results suggest that MTCA may be one of the age-related fluorogens. In vitro experiments revealed that MTCA, a condensation reaction product of tryptophan with acetaldehyde, can be formed in the mixture of tryptophan and glucose by low temperature heating. Furthermore, the MTCA levels in the homogenate of cataractous lenses were significantly increased with the heating time, when the homogenate without addition of glucose and tryptophan was heated at 60°C. This indicates that enough material such as tryptophan for MTCA formation is present in human cataractous lenses. Therefore, it may be that MTCA is formed in human lens. However, we can not rule out the possibility that MTCA in the lens may be derived from other sources, and deposited in the lens since MTCA is reported to be present in various foodstuffs and alcoholic beverages.