Use of mass spectrometry to measure aspartic acid racemization for ageing beluga whales

Abstract

We developed a novel analytical method to measure the D‐ and L‐isomers of aspartic acid ( AA ) in the eye lens of beluga whales ( Delphinapterus leucas ) for age determination. The method was based on hydrolysis of the eye lens under acidic conditions followed by direct injection onto a Chirobiotic T (25 cm × 4.6 ID , 5 μm particle size) high performance liquid chromatography analytical column and detection by tandem mass spectrometry operated in the negative ionization mode. The detection limit of the method was 550 pg for each isomer, the repeatability expressed as the relative standard deviation was 8% and the linear dynamic range was from 0.05 mM to 1 mM . The validated method was used to estimate, for the first time, the rate of racemization ( K asp ) of the two AA isomers and also the ratio of D/L at age 0, (D/L) 0 , in 34 beluga whales from the Canadian Arctic. At a mean ocular lens temperature of 17.8°C, respective K asp and (D/L) 0 were 3.48 ± 1.47 × 10 −3 /yr and 0.010 ± 0.005. We evaluated factors that impact K asp and affect uncertainty in age estimation and outline the steps required to incorporate the method in wildlife management decisions.

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