An intermediate in the synthesis of glucobrassicins from 3-indoleacetaldoxime by woad leaves

Abstract

Leaves of woad (Isatis tinctoria L.) were found to incorporate efficiently tritiated indoleacetaldoxime and (35)S from (35)S-l-cystine into glucobrassicin and sulfoglucobrassicin. Time course of incorporation of (35)S from (35)S-cystine into the glucosinolates indicated that glucobrassicin was formed first and then sulfoglucobrassicin. Simultaneous administration of tritiated indoleacetaldoxime and (35)S-cystine gave doubly labeled glucobrassicin and sulfoglucobrassicin. About twice as much (35)S was present in sulfoglucobrassicin as compared to glucobrassicin per unit of (3)H incorporated, indicating that a second, probably oxidized, atom of (35)S was later introduced into sulfoglucobrassicin. However, the (35)S incorporated from cystine into both glucosinolates during the first 8 hours of metabolism was almost exclusively in the divalent sulfur moiety. The incorporation patterns of (35)S and titritated indoleacetaldoxime into the glucosinolates suggested a fast turnover of glucobrassicin in the metabolizing leaves.A new indolic, sulfur-containing neutral compound X was found to accumulate in woad leaves when administered (3)H-3-indoleacetaldoxime and cold cystine or (35)S-cystine and cold 3-indoleacetaldoxime. This accumulation was enhanced about 2- to 2.5-fold by the simultaneous administration of postassium selenate, an inhibitor of biological sulfation processes. Selenate also appeared to inhibit the conversion of glucobrassicin to 1-sulfoglucobrassicin. Partially purified compound X was efficiently converted (56-60%) to glucobrassicin and 1-sulfoglucobrassicin on readministration to woad leaves, indicating it to be a precursor of the glucosinolates. Compound X, on treatment with myrosinase, slowly yielded a less polar, indolic, sulfur containing compound Y and glucose. Compound Y decomposed with time into indoleacetonitrile suggesting that it may be indoleacetothiohydroximate. Compound X has been tentatively assigned the structure of desthioglucobrassicin, the nonsulfated form of glucobrassicin.