Gj. Tanner et al., PROANTHOCYANIDINS INHIBIT HYDROLYSIS OF LEAF PROTEINS BY RUMEN MICROFLORA IN-VITRO, British Journal of Nutrition, 71(6), 1994, pp. 947-958
Proanthocyanidins (condensed tannins; PA) purified from the leaves of
forage legumes Trifolium arvense, Lotus pedunculatus, Lotus corniculat
us, Dorycnium rectum, Coronilla varia, Onobrychis viciifolia, or Hedys
arum coronarium, were added to soluble lucerne (Medicago sativa) leaf
protein and incubated with strained rumen fluid in vitro. Fractions we
re collected and frozen immediately. Denatured proteins were fractiona
ted by sodium dodecylsulphate-polyacrylamide gel electrophoresis (SDS-
PAGE), stained, and relative levels were quantified by densitometry. I
n the absence of PA the large subunit (LSU) of ribulose bisphosphate c
arboxylase (EC 4.1.1.39) was susceptible to proteolysis by rumen micro
flora but the small subunit (SSU) resisted breakdown, PA purified from
Onobrychis was added to soluble leaf protein, at PA:protein ratios be
tween 1:1 and 1:20. The rate of proteolysis of LSU was significantly r
educed at PA:protein ratios of 1:2 and 1:1 (P < 0.001) and the rate of
digestion was reduced by between 3- and 21-fold. In separate experime
nts PA isolated from the range of species described was added to rumen
fluid to give PA:protein ratios of 1:5. The addition of PA significan
tly reduced the rate of proteolysis of LSU,,when compared with PA-free
control. There were only small differences between PA from different
species. The inhibitory effect of PA may have been due to PA binding t
o the dietary protein or to the rumen proteases, interfering with the
action of proteases on susceptible sites within the substrate.