Radioimmunoassay for the determination of loganin and the biotransformation of loganin to secologanin by plant cell cultures

A radioimmunoassay technique has been developed for the quantitative measurement of loganin in crude extracts from both fresh and dried material of whole plants and cultivated plant cells. The assay makes use of 6′-carboxyloganin which is rendered immunogenic through linkage to bovine serum albumin. The tracer molecule was synthesized via periodate opening of the glucose moiety of loganin and subsequent reduction with sodium borotritide of high specific activity. The rabbit antibodies had a high affinity (Ka = 1.6 × 1091/mol) for loganin and permitted the detection of as little as 0.1 ng per 0.05 ml of sample. The antiserum was highly specific for loganin and its aglycone, with only 10-hydroxyloganin and 7-epi-loganin showing a substantial cross reactivity. A number of cell cultures of the Caprifoliaceae were tested for their ability to transform added loganin to secologanin. By labelled precursor feeding experiments members of the genera Weigelia, Lonicera, Hydrangea and Symphoricarpus were found to open the cyclopentane ring of loganin. The time course of the biotransformation of loganin was monitored using radioimmunoassays for both loganin and secologanin and cell cultures of Lonicera tatarica as biological material.