Condensed tannins (CT) or proanthocyanidins (PA), which occur in a restricted range of forages, have the ability to interact with proteins and enzymes and can inýuence the digestion of plant protein in the rumen. We compared the eþ ects of CT extracts from Lotus corniculatus and pedunculatus on degradation of the principal leaf protein, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), by rumen microorganisms. Total soluble leaf protein extracted from white clover (Trifolium repens) was incubated with fresh rumen ýuid from sheep and a range of concentrations of each CT extract. The rate of degradation of the large (LSU) and small subunit (SSU) of Rubisco was quantiüed by fractionating the proteins in samples taken from in vitro rumen incubations using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and imaging densitometry. To deduce the eþ ects of the CT extracts, experiments were performed in the presence (CT inactive) and absence (CT active) of polyethylene glycol (PEG ; MW 3350). The two CT extracts diþ ered markedly in their eþ ects on the degradation of the LSU and SSU of Rubisco. At concentrations of 0.89 and 1.79 mg CT mg-1 total soluble leaf protein, the CT extract from L pedunculatus was more eþ ective at preventing the degradation of the LSU and SSU by rumen microorganisms than the CT extract from L corniculatus. At a concentration of 1.79 mg CT mg-1 total soluble leaf protein, the CT extracts from L corniculatus and pedunculatus prevented about 0.75 and 0.83 of the LSU and about 0.69 and 0.86 of the SSU, respectively, from being degraded. Addition of PEG removed the inhibition and almost complete degradation of these proteins occurred, as was the case in incubations without CT extracts.
The results of this study suggest that the concentration of CT in the diet and the chemical structure which aþ ects the activity of the CT needs to be considered when assessing the eþ ects of CT on protein metabolism in ruminants.