In-vitro rates of rumen proteolysis of ribulose-1,5-bisphosphate carboxylase (Rubisco) from lucerne leaves, and of ovalbumin, vicilin and sunflower albumin 8 storage proteins

Laboratory systems were developed, based upon in-vitro incubations with rumen fluid, to exam the rate of proteolysis (ie degradation) of sunflower albumin 8 (SFA 8; 24% sulphur amino acids; SAA) from sunflower seeds, ovalbumin (6% SAA) from chicken egg white, ribulose-1,5-bisphosphate carboxylase (Rubisco; 3% SAA) from lucerne leaves, and vicilin (0% SAA) from pea seeds. After fractionation by SDS-PAGE, proteins were analysed by either Western blotting, using specific antibodies (SFA 8, vicilin and ovalbumin) or by Coomassie blue staining (Rubisco). Proteolysis of the large subunit of Rubisco occurred very quickly and as two components, with half-lives of 11.6 and 1.5 h. The small subunit of Rubisco was more resistant to degradation, with a half-life of 17.3 h. Vicilin was degraded extremely rapidly (half-life 0.16 h). SFA 8 and ovalbumin both showed resistance to degradation, but by two different mechanisms. Ovalbumin was not degraded at all during the initial 16 h of incubation, but then degraded with a half-life of 8.7 h. SFA 8 (mol wt 12100) disappeared very rapidly, with a half-life of 3.0 h. The degradation of SFA 8 was associated with the appearance of a polypeptide (mol wt 8000), which was extremely resistant to degradation, with a half-life of 69.3 h. It was concluded that both the number of disulphide linkages and tertiary structure were important in determining resistance of proteins to rumen degradation, and that incorporation of SFA 8 and ovalbumin proteins into forages using genetic engineering techniques would be likely to increase the quantity of SAA bypassing rumen fermentation.