The Development of a Plasma Volume Expander 69 



Of considerable importance in connection with the use of these 

 materials is the question of their metabolic fate in man. 



It has been ascertained that both the peptide and the glutamyl 

 peptide conjugates are broken down by aqueous tissue extracts of 

 practically all human tissues including red cells, kidney, liver, spleen, 

 and brain. The major exception seems to be muscle. It is significant 

 also that plasma does not break down either peptide or conjugate. 

 The precise extent of breakdown that occurred in these tissues in vitro 

 is not known. It has been observed that if peptide or conjugate solu- 

 tions are exposed to homogenates of the organs, or hemolyzates, then 

 70 or SO/! of the safranine-precipitable conjugate or peptide disappears 

 within 12 hours at room temperature. Since it is known that, under 

 the conditions of assay used, safranine precipitates quantitatively pep- 

 tides and conjugates of molecular weights of 3000 and higher, it is 

 obvious that the material that has disappeared in this time must have 

 been degraded to molecules of less than 3000 molecular weight. By 

 microbiological assay and chromatographic analysis it was determined 

 that after precipitation with trichloracetic acid there were present in 

 the digestion mixture low molecular weight glutamyl peptides and some 

 free glutamic acid. The free glutamic acid found was never more than 

 10^ of the conjugate or peptide originally present, but it is possible 

 that some free glutamic acid produced may have been metabolized. 

 This point of completeness of metabolism or excretion of the material 

 is important and will have to be conclusively settled by isotopic anal- 

 ysis. It is interesting that, under the same conditions used in obtaining 

 the above data, synthetic a-linked glutamyl peptide is degraded ex- 

 tremely slowly, if at all, by extracts of human kidney, liver, and red 

 cells. 



The potentialities for control of molecular size of conjugate may be 

 exemplified by the following. In three separate preparations where 

 the size of the side chain and the ratio of side chain to azide were kept 

 constant, but where the backbone varied in length in the approximate 

 ratio of 1 :2:3, the half-lives of the material in the human blood stream 

 were respectively 12, 18, and 24 hours. In another set of preparations 

 when backbone size was kept constant it was found that enlargement 

 by side chains consisting of glutamic acid (or various other amino 

 acids i caused no significant increase in the half-life in the blood stream 

 over that of the original backbone peptide, whereas side chains of 

 molecular weight of a few thousand increased the half-life by many 

 hours even though the number of amino acid side chains per backbone 

 was higher than that of peptide side chains. In a third instance where 



