146 APPENDIX C 



temperatures. The difference is that in living 

 organisms such reactions are catalyzed by en- 

 zymes. All known enzymes are proteins, and 

 hence possess typical protein properties. 



In vertebrates generally, the pancreas secretes 

 into the digestive system a number of enzymes 

 (trypsin, chymotrypsin, etc.) that catalyze the 

 breakdown of proteins into smaller units, called 

 peptides, and individual amino acids. You will 

 be supplied with a dilute pancreatic extract in 

 order to carry out at room temperature the 

 digestion of the casein you have prepared. 



Measure about 0.2 gm of casein into a test 

 tube. Add approximately 2 ml of O.^T; pan- 

 creatic extract, dissolved in 0.1 M phosphate 

 buffer at pH 7.0. Swirl the contents of the test 

 tube to dissolve or suspend the casein. Add a 



small crystal of thymol to prevent the growth of 

 microorganisms. Label and initial your tube, 

 stopper it, and give it to your instructor to store 

 until the next laboratory period. 



A >vord on molecular structure 



Read the material on this topic in Exercise III 

 and make yourselves models of a representative 

 fat, and the sugars glucose and galactose. See 

 what it means to join glucose and galactose to- 

 gether, taking out a molecule of water, to yield 

 lactose. Similarly construct a polypeptide chain 

 from a few generalized amino acids, and see what 

 it means to insert a molecule of water so as to 

 break (hydrolyze) such a chain, the process 

 catalyzed by such protein-hydrolyzing enzymes 

 as are found in pancreatic extracts. 



PART 2 



(Readings: G. Wald, "The Origin of Life," Sci. Am. 191, No. 2, 45-53, Aug. 

 1954, Reprint No. 47. P. Doty, "Proteins," Sci. Am. 197, No. 3, 173-184, 

 Sept. 1957, Reprint No. 7. E. O. P. Thompson, "The Insulin Molecule," 

 Sci. Am. 192, No. 5, 36^1, May 1955, Reprint No. 42.) 



Shake the tube containing the lactose crystals, 

 and filter with suction. Wash them with two 

 5-ml portions of acetone. (Acetone is a fire 

 hazard. When you are through, flush it down 

 the drain with plenty of water.) Suck the crys- 

 tals dry, and transfer them to a cardboard 

 container. 



We will now examine some of the specific 

 properties of the substances isolated from milk. 

 Record all observations immediately. First note 

 the colors and textures of these substances. 

 Then carry out the following general tests for 

 protein and sugar on each of the four com- 

 ponents. 



size of a BB) of casein in one, of coagulated 

 protein in another, of lactose in a third and Ca 

 phosphate (or oxalate) in the last. 



Biuret test for proteins 



Add 3 ml of sodium hydroxide to each of one 

 series of test tubes. Gentle warming of a tube 

 in a water bath will hasten the solution of 

 whatever material it contains, but run the rest 

 at room temperature. Add a few drops of 

 copper sulfate solution. Note any appearance 

 of color. The biuret test is given not only by 

 proteins, but by any substance that contains 

 so-called peptide bonds ( — CO — NH — ). 



EXPERIMENTAL PROCEDURE 



Prepare two groups of test tubes, with five in 

 each series. Leave one tube of each series 

 empty to serve as a blank. In the other four 

 tubes of each series place a pinch (about the 



Benedict test for sugars 



Add 5 ml of Benedict solution to each test 

 tube in the second series. Hold them in the 

 boiling water bath for 2 minutes. Compare the 

 colors. The Benedict test is given by all sugars 

 that contain groups (aldehyde or ketone) that 



