ALPHA-TOMATINE DETERMINATION IN TOMATOES BY HPLC USING PULSED AMPEROMETRIC DETECTION

As part of a program to control the biosynthesis of glycoalkaloids, we used an improved HPLC method with pulsed amperometric detection (PAD) to measure the alpha-tomatine content of store-bought and field-grown, including transgenic, red, and green tomatoes. The HPLC method responded linearly to alpha-tomatine in the range 0.125-12.5 mu g, suggesting a lower limit of detection of about 125 ng of alkaloid. Recoveries from tomato extracts spiked with alpha-tomatine ranged from 97 to 107%. alpha-Tomatine and a new glycoalkaloid tentatively identified as dehydrotomatine, whose molecular mass determined by mass spectrometry is 2 Da less than that of alpha-tomatine, separated well on the HPLC column. The ratio of alpha-tomatine to dehydrotomatine in commercially available standards was approximately 10:1. The alpha-tomatine content of ripe red tomatoes ranged from 0.03 to 0.6 mg/100 g of fresh weight. The corresponding values for unripe green tomatoes ranged from 4 to 17 mg/100 g of fresh weight. These results show that the ratio of alpha-tomatine content for the highest concentration in green tomatoes to the lowest value in red tomatoes is more than 500. The difference in alpha-tomatine content of transgenic and nontransgenic tomatoes, at the same level of ripeness, was negligible. Possible applications of the HPLC-PAD method are discussed.