The Availability of the Proteins and Inorganic Salts of the Green Leaf

Abstract
A study of the known in vitro digestion technics failed to produce a practical method that might be used to determine the utilizable nitrogen in the green leaf. The results obtained, however, suggested that a closer approximation to the available nitrogen could be obtained by eliminating the fat soluble nitrogen, by means of extraction with an alcohol-ether mixture, and by boiling off the nitrate, ammonia and amide nitrogen from the substance to be analyzed before determining protein and amino acid nitrogen. On this basis a simple procedure for estimating the available nitrogen present has been devised. An investigation of the digestion of spinach by successive treatment with pepsin, trypsin and erepsin was made. The results obtained when the entire leaf was digested are similar to those obtained with pure proteins. This indicates that with certain materials, at least, it is possible to perform exact proteolytic digestions without necessarily isolating the protein. The application of in vitro digestions to determine the availabilities of the calcium, iron, chloride and phosphorus in spinach confirmed the view advanced in the literature that the calcium and iron in spinach is only partly available. Enzymatic digestion brought about 30% of the calcium and 40% of the iron into solution. Chloride and phosphorus were readily dissolved in water alone.