Turnip (Brassica rapa ssp. rapa) and rape (Brassica napus ssp. biennis) and other brassica forage crops are regarded as "safe" feed for cattle during late summer and fall in the North Island of New Zealand when high Pithomyces chartarum spore counts in pastures frequently lead to
sporidesmin toxicity (facial
eczema). Sporadic acute severe cases of turnip
photosensitization in dairy cows characteristically exhibit high γ-glutamyl
transferase and
glutamate dehydrogenase serum
enzyme activities that mimic those seen in facial
eczema. The two diseases can, however, be distinguished by histopathology of the liver, where lesions, in particular those affecting small bile ducts, differ. To date, the hepato-/cholangiotoxic
phytochemical causing liver damage in turnip
photosensitization in cattle is unknown. Of the hydrolysis products of the various
glucosinolate secondary compounds found in high concentrations in turnip and rape, work has shown that
nitriles and epithionitriles can be hepatotoxic (and nephro- or pancreatotoxic) in rats. These derivatives include β-hydroxy-
thiiranepropanenitrile and 3-hydroxy-4-pentenenitrile from
progoitrin;
thiiranepropanenitrile and
4-pentenenitrile from
gluconapin; thiiranebutanenitrile and 5-hexenenitrile from glucobrassicanapin; phenyl-3-propanenitrile from
gluconasturtiin; and
indole-3-acetonitrile from
glucobrassicin. This perspective explores the possibility of the preferential formation of such derivatives, especially the epithionitriles, in acidic conditions in the bovine rumen, followed by absorption, hepatotoxicity, and secondary
photosensitization.