Metabolite profiling of sesquiterpene lactones from Lactuca species - Major latex components are novel oxalate and sulfate conjugates of lactucin andits derivatives

Wounding leaves or stems of Lactuca species releases a milky latex onto the lent surface. We have examined the constituents of latex from Lactuca sati va (lettuce) cv, Diana, The major components were shown to be novel 15-oxal yl and 8-sulfate conjugates of the guaianolide sesquiterpene lactones, lact ucin, deoxylactucin, and lactucopicrin. The oxalates were unstable, reverti ng to the parent sesquiterpene lactone on hydrolysis, Oxalyl derivatives ha ve been reported rarely from natural sources, The sulfates were stable and are the first reported sesquiterpene sulfates from plants. Unusual tannins based on 4-hydroxyphenylacetyl conjugates of glucose were also identified. Significant qualitative and quantitative variation was found in sesquiterpe ne lactone profiles in different lettuce varieties and in other Lactuca spp , The proportions of each conjugate in latex also changed depending on the stage of plant development. A similar profile was found in chicory, in whic h oxalyl conjugates were identified, but the 8-sulfate conjugates were nota bly absent. The presence of the constitutive sesquiterpene lactones was not correlated with resistance to pathogens but may have a significant bearing on the molecular basis of the bitter taste of lettuce and related species. The induced sesquiterpene lactone phytoalexin, lettucenin A, was found in the Lactuca spp, but not in chicory.