Solubilization and degradation of ribulose-1,5-bisphosphate
carboxylase/oxygenase (EC 4.1.1.39;
Rubisco) protein from white clover (Trifolium repens)
and Lotus corniculatus by rumen microorganisms and
the effect of condensed tannins on these processes
Authors: MIN B. R. , MCNABB W. C. , BARRY T. N. , PETERS J. S. 
Published: July 2002

In situ and in vitro rumen incubations were used to determine the effect of condensed tannins (CT)
on the solubilization and degradation of the plant protein from white clover (Trifolium repens) and
Lotus corniculatus. These forages contained, respectively 0·3 and 22·1 g CT/kg dry matter (DM). The
sheep used for the experiments were also fed either white clover or L. corniculatus. Effects of CT were
determined by making measurements in the presence and absence of polyethylene glycol (PEG;
molecular weight 3500), which binds and inactivates CT. The loss of DM, neutral detergent fibre
(NDF), total nitrogen (N) and Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase; EC
4.1.1.39; fraction I leaf protein) from polyester bags suspended in the rumen of sheep was measured.
The loss of these constituents from polyester bags suspended in the rumen was used as a measurement
of their solubilization. Degradation was defined as the disappearance of Rubisco from white clover
and L. corniculatus added to in vitro incubations with rumen fluid obtained from the same fistulated
sheep fed either white clover or L. corniculatus.

In the absence of PEG, the in situ loss of Rubisco from L. corniculatus was less rapid than the loss
of this protein from white clover when each forage was incubated in the rumen of sheep fed the same
diet. Addition of PEG tended to increase the loss of Rubisco from L. corniculatus, suggesting that CT
slowed the rates of solubilization of Rubisco from this forage. Effects of rumen fluid were small, but
there was some evidence that the rumen fluid in sheep fed L. corniculatus reduced the solubilization
of Rubisco from white clover. The action of CT did not inhibit the in situ loss of NDF from either
white clover or L. corniculatus.

In the absence of PEG, the in vitro degradation of Rubisco from L. corniculatus was slower when
compared to the degradation of this protein from white clover; PEG addition increased the
degradation of Rubisco from L. corniculatus, but not from white clover, showing that CT was the
causal agent. The addition of CT extracted from L. corniculatus markedly depressed the degradation
of Rubisco from white clover, with the effect being completely reversible by PEG. The large subunit
(LSU) of Rubisco was consistently degraded at a faster rate than the small subunit (SSU) and added
CT had a greater effect in slowing the degradation of the LSU compared to the SSU. There was little
difference in the degradation of Rubisco when rumen fluid from sheep fed either white clover or L.
corniculatus was used for in vitro incubations.

It was concluded that the action of CT from L. corniculatus reduces the digestion of protein in the
rumen of sheep. This effect is predominantly due to the action of CT reducing the degradation of
plant protein, although CT also reduced the solubilization of plant protein. The main effects of CT
on protein solubilization and degradation seemed to be produced locally by CT present in plant
tissue; transfer of these effects through rumen fluid was small in magnitude.