1. Field of the Invention
The present invention relates to a method for preparing Fraction I protein from unpurified extracts of a variety of plant species by crystallization with polyethylene glycol, and more particularly to a method which provides for the removal of certain phosphorylated sugars from the polyethylene glycol solution and allows recycling of the solution.
2. Description of the Prior Art
Fraction I protein, the most abundant protein found in plants, has been identified as the enzyme ribulose 1, 5-bisphosphate carboxylase (RuBisCO). Fraction I protein is widely distributed in nature and constitutes up to 50% of the water soluble protein contained in leaves and approximately 20% of the total plant protein. The amino acid composition of Fraction I protein is well balanced in terms of the essential and non-essential amino acids, comparing favorably with soybean, casein, and animal proteins. The amounts of the essential amino acids in Fraction I protein, with the exception of methionine, meet and often exceed most published human nutritional requirements.
For some years, it has been the goal of a number of scientific and industrial researchers to provide a large-scale method for recovering Fraction I protein from a variety of plant sources. Much work has been done, and substantial steps have been taken toward this goal. See, e.g., Paulsen and Lane (1966) Biochem. 52:2350-2357, describing a method for purifying spinach RuBisCO on a laboratory scale. Compare the more recent work described in U.S. Pat. Nos. 4,347,324 and 4,268,632 to Wildman et al., U.S. Pat. No. 3,340,676 to Bourque, and particularly U.S. Pat. Nos. 4,334,024 and 4,400,471 to Johal, the inventor herein.
While the methods taught in these references hold much promise, until the methods can be improved to provide for the efficient production of Fraction I protein on a commercial scale, their promise cannot be realized. For example, heretofore, recycling of polyethylene glycol (PEG) used to crystallize the RuBisCO has been difficult because of the presence of other substances released during fractionation of the plant material. Thus, it would be highly desirable to provide improved methods for the recovery of Fraction I protein from various plant sources on a large scale with relatively low costs, particularly methods which allow recycling of the PEG.