COMPARISON OF THE PRECIPITATION OF ALFALFA LEAF PROTEIN AND BOVINE SERUM-ALBUMIN BY TANNINS IN THE RADIAL DIFFUSION METHOD

The precipitation of protein by condensed and hydrolysable tannins was evaluated with the radial diffusion method of Hagerman (1987) using b ovine serum albumin (BSA) and isolated leaf protein from fresh alfalfa (Medicago sativa). Alfalfa leaf protein (AALP) was included at two co ncentrations, 25 and 156 mg N litre(-1), at pH 6.8 and 39 degrees C to simulate rumen conditions. The condensed tannins were purified from l yophilised samples of Arachis pintoi, Desmodium ovalifolium, Gliricidi a sepium, Manihot esculenta and quebracho (Schinopsis balansae). Hydro lysable tannins from tannic acid (TA) were used as well. There was a s ignificant interaction (P < 0.001) between tannin and protein source, and protein level on protein precipitation. Most purified condensed ta nnins (CTs) precipitated more AALP than BSA when protein was included at the same level. Purified CT from quebracho and hydrolysable tannin from TA failed to precipitate AALP at both protein levels. In a second experiment, tannins from crude plant extracts were studied in the rad ial diffusion method using BSA and two levels of AALP. The crude plant extracts were obtained from lyophilised plant samples of A pintoi, Ce ntrosema macrocarpum, Clitoria ternatea, D ovalifolium, Erythrina bert eroana, E poepigiana, G sepium, M esculenta, Pueraria montana and P ph aseoloides. The protein precipitated by soluble tannins in the plant s amples was correlated to the total phenolic content and to the soluble CT estimated by the acid butanol assay or by the radial diffusion met hod. Tannins from different plant species precipitated different amoun ts of BSA and AALP. Therefore, the measures of the biological activity of tannins based on BSA precipitation may not reflect the ability of tannins to precipitate proteins of plant origin such as those commonly found in the diets of herbivores. The present study offers the possib ility of using the radial diffusion method with plant proteins at prec ipitation conditions similar to those in the rumen.