During protein depletion produced by plasmapheresis and a very low protein diet there is a proportionately greater decrease in extravascular, extracellular fluid protein than in plasma protein. A shift in the normal ratio of protein in these 2 compartments, approximately 1 to 1 in the dog, to over 2 to 1 as a result of depletion indicates an important, labile source of reserve protein for the plasma in the interstitial fluids. This reserve source is limited since a maximum drop of 50 per cent in the total exchangeable pool and of 75 per cent in the extravascular, extracellular protein occurred after both shorter and longer periods of depletion. Under the rigid conditions of these experiments additional plasma protein removal was associated with loss of weight despite adequate caloric intake. Investigation of the status of the interstitial fluid proteins in other conditions associated with disturbed protein metabolism seems warranted.

The rate of plasma protein turnover is more rapid in dogs receiving adequate dietary protein than when a diet devoid of protein is fed. Both albumin and combined globulins are involved in this change. The difference in turnover is reflected in a total protein half-life of 4.8 days with protein feeding versus 7.8 days without protein in the diet and in the metabolism of 1.0 and 0.65 gm. per kilogram of body weight per day on the respective diets. Additions of dietary protein from 10 to 30 per cent caused no further increase in the rate of plasma protein turnover. With protein depletion due to plasmapheresis and a very low protein diet there is evidence of reduced protein metabolism as indicated by nitrogen retention as well as a reduction in total plasma protein breakdown and interchange of isotope between plasma and tissue proteins. Following introduction of labeled plasma protein into the circulation the net amount of isotope transferred to tissues has been computed from the difference between total plasma protein breakdown and combined C 14 excretion in urine and expired air. In animals receiving adequate dietary protein, tissue transfer amounts to 70 per cent of the total lost from the plasma proteins each day while the percentage rises to 85 in depleted dogs deprived of protein. In dogs with both plasma and tissue proteins labeled it can be estimated that, under conditions of protein feeding, an amount of C 14 approximately equal to that lost from the plasma must recycle to account for the observed decrease in Apparent plasma protein turnover rate, ( t ½ of 15 versus 5 days). Without protein in the diet the isotope contribution of the tissues to the maintenance of plasma protein levels must be as great as or greater than that transferred in the opposite direction.

Red cell stroma protein and hemoglobin can be labeled by feeding C 14 lysine during periods of active blood regeneration following anemia. Stroma proteins are produced and a maximum concentration of the C 14 label appears 2 to 3 days earlier than with hemoglobin,—which is to say that stroma building precedes hemoglobin construction. The concentration of isotope in stroma protein may exceed its concentration in hemoglobin during regeneration following anemia due to blood loss. Diets favorable for hemoglobin regeneration may force the hemoglobin isotope concentration above that of the stroma protein. In hemolytic anemias great reserves of red cell building material are stored in the body. These stores may modify the curves of isotope concentration in red cells during the recovery periods. When finally formed, the mature red cells show little or no evidence of participation in general body protein metabolism during their life in the circulation.

Plasma proteins tagged in vivo by feeding D-L -lysine-ϵ-C 14 to donor dogs have been administered to pregnant dogs by both oral and intravenous routes. A relatively small percentage of the C 14 activity originally incorporated in these proteins is found to pass from mother to fetus after intravenous injection. The amount transferred tends to increase with the length of gestation period and total number of fetuses. Plasma protein labeled with I 131 does not cross the placenta in the dog, but does in the rabbit. Evans blue dye does not cross the placenta of the dog. After oral administration of labeled plasma protein or lysine, C 14 is transferred promptly and in considerable quantity to the fetus. Labeled plasma proteins disappear more rapidly from the circulation of pregnant than of normal dogs. This increased metabolic turnover occurs without excretion of any excess waste metabolites. The chorionic epithelium , gram for gram, is probably 2 to 3 times as active as the hepatic epithelium in protein metabolism. These findings indicate an important placental function related to maternal and fetal protein metabolism. While the placenta utilizes maternal plasma proteins and amino acids, in a quantitative sense the latter appear to supply the major nitrogen needs of the growing fetus.

Carbon-14-labeled plasma proteins given by mouth to dogs with sterile abscesses undergo decreased absorption, presumably owing to impaired digestion of protein. The turnover of plasma albumin is greatly accelerated but the globulins, excluding fibrinogen, show little change during the acute stage of the sterile inflammation. Fibrinogen shows very rapid production and utilization during acute inflammation. Large amounts of C 14 are incorporated in fibrinogen within a few hours after ingestion of the labeled material. The labeled fibrinogen largely disappears within 2 to 4 days after its production. The appearance of C 14 in new red cells from labeled protein or amino acid sources is reduced by inflammation—evidence of impaired synthesis. The pus of the sterile abscess contains a good deal of C 14 activity which at times is as much as that found in the liver. Pus cell C 14 activity per milliliter is similar after injection of labeled plasma and ingestion of labeled plasma or lysine. However, the pus cell fraction contains 3 to 4 times more C 14 activity per milliliter than does the supernatant fluid when the isotope is fed. In the supernatant fluid the activity is all within precipitable protein, much of which is probably derived from the blood plasma. In spite of increased loss of C 14 as CO 2 in the expired air and in the pus, there is evidence of conservation of protein-building materials for maintenance of new plasma proteins and tissue proteins in the more active organs ( e.g . liver)—a shift of protein C 14 from the less active tissues (muscle and skin).

The metabolism of homologous plasma proteins, labeled with lysine-ϵ-C 14 , after oral administration to dogs has been investigated. The speed of the various processes involved is indicated by the maximum rate of C 14 O 2 excretion which is attained within 1 to 4 hours, the prompt appearance of protein activity in the plasma and disappearance of non-protein activity from it, both virtually complete in 7 to 10 hours, as well as the rapid incorporation of a large percentage of the fed-C 14 into tissues. There are no essential differences between the behavior of labeled plasma and that of an amino acid digest containing ϵ-C 14 labeled lysine when these two materials are given orally. At the end of 48 hours after labeled plasma feeding, a CO 2 elimination of 16 to 28 per cent of the fed C 14 is noted. In contrast, after 48 hours following labeled plasma by vein, a CO 2 elimination of only 2.5 per cent is recorded—almost a 10 to 1 ratio. We believe this, together with the data concerning plasma and tissue protein activity, represents a significant difference in the metabolic process. The evidence favors a complete breakdown of plasma protein to the amino acid level when given orally but not when given by vein.

Plasma containing carbon 14 -labeled albumin and globulin, obtained by feeding ϵ-C 14 D,L .-lysine to a donor dog, has been injected intravenously and intraperitoneally into recipient dogs with experimental ascites. The circulation and interchange of total plasma protein between circulating blood and ascitic fluid have been confirmed and the participation of both albumin and globulin in this interchange has been demonstrated. Labeled albumin tends to reach equilibrium in plasma and ascitic fluid in a shorter period of time (1 to 2 days) than does globulin (2 or more days), after administration of labeled plasma by either route. Evidence is presented that the rate of transfer of albumin across the peritoneal membrane is at least three times faster than that of globulin in terms of weight.

Labeled plasma proteins obtained from donor dogs, previously fed ϵ-C 14 - dl -lysine, have been given intravenously to recipient dogs. The disappearance of labeled globulin from the plasma at a rate considerably faster than albumin has been confirmed. Evidence suggesting that the mass of protein in solution in the extravascular, extracellular fluid is approximately equal to the plasma proteins in circulation has been derived from a study of the dilution of labeled plasma protein by repeated injections of non-labeled plasma protein. In a period of 7 days the transfer of C 14 from plasma to tissue proteins amounted to between 30 and 40 per cent of the activity in the labeled plasma protein injected intravenously. The conversion was accompanied by a very small loss of activity in the urine and expired air and the activity remained in the lysine residue of the liver and probably of other tissues. The data presented favor the view that plasma proteins are utilized in the body economy after partial catabolism within the cell area and provide no evidence of complete breakdown to the amino acid level.

A dog, doubly depleted of blood cells and plasma proteins, was fed dl -lysine labeled with C 14 in the epsilon carbon position. In the first 8 hours 28 per cent of the administered C 14 was excreted in the urine; in the first 72 hours, 35 per cent. Twenty-four hours after feeding, 4.2 per cent of the fed C 14 was circulating in the plasma, decreasing to 1 per cent at the end of 17 days. The C 14 content of the blood cells increased from 1 per cent at 24 hours to 5.5 per cent in 5 days and 6.8 per cent in 22 days. Evidence based on the rate of decrease of the C 14 content of circulating blood cells is presented indicating an average life of 115 days for the erythrocyte protein as an entity not interchanging with extracellular constituents. This corresponds closely to the life span of the dog erythrocyte, 112 to 133 days according to the best evidence otherwise available and indicates that this experiment has actually measured the life span of the dog erythrocyte. Following breakdown of blood erythrocytes the protein comprising them is not used preferentially for the formation of new erythrocytes.

Racemic lysine labeled with C 14 in the epsilon carbon position was fed to dogs. The distribution of C 14 in blood and tissue fractions is recorded. In normal dogs sacrificed at 24 hours, approximately one-third of the C 14 was found in the urine, one-third in expired air, and one-third in the body, mostly in protein, predomantly as lysine residues. The rate of C 14 excretion as CO 2 , hour by hour, paralleled closely the amount of non-protein C 14 in the blood plasma. The liver, kidney, pancreas, and spleen all have high values for C 14 in 24 hour and 17 day experiments. The gastrointestinal tract is significantly high in the 24 hour experiments. Plasma protein from animals previously fed C 14 containing lysine and thus in turn labeled, was transfused into other dogs and the rate of disappearance of albumin and globulin fractions from the circulation of the recipient dog followed. The results lead to the conclusion that as a whole, plasma proteins are utilized and replaced at a rate of at least 10 per cent per 24 hours. This minimum rate is substantially faster than turnover rates commonly accepted and emphasizes the rôle played by the plasma proteins in the protein economy of the body. The exact rate determination is made uncertain by the lack of knowledge of the magnitude of the amount of protein in solution in extracellular and lymph spaces and its rate of equilibrium with circulating plasma proteins. Evidence from these transfusion studies indicates that plasma globulin is metabolized at a significantly faster rate than plasma albumin. This is confirmed by the observation that following the feeding of labeled lysine to dogs, C 14 is first incorporated in globulin in high concentration but that later it also disappears more rapidly from the globulin fraction. These data suggest that the period of bone marrow maturation of the red cell during which time its related hemoglobin is synthesized does not exceed 3 to 5 days.

Sterile inflammation induced by repeated subcutaneous injections of turpentine in non-anemic, non-iron—deficient dogs, leads to a fall in plasma iron concentration, the development of a moderate anemia, and a marked delay in the uptake by the red blood cells of intravenous radioiron. Similar periods of inflammation in anemic, iron-deficient dogs on a diet low in iron cause no increase in the degree of anemia and no inhibition of red blood cell uptake of intravenous radioiron. Radioiron appears only in traces in abscess exudates. Intravenous iron disappearance curves following a single injection are uninfluenced by sterile inflammation in either anemic or non-anemic dogs. The impairment of hemoglobin synthesis caused by inflammation is at most a relative matter, since the anemia that develops is seldom severe or progressive, and since the inhibition can be overcome if the marrow is sufficiently stimulated by the demands of a severe continuing anemia.

Lowered plasma protein concentration following single injections of gum acacia in the dog is due in some part to dilution, and in greater part to actual decrease in total circulating protein. The maximum decrease in the total circulating protein does not take place at the same time as the maximum decrease in concentration. Fluctuations in fibrinogen concentration are marked, and are not necessarily proportional to changes in plasma protein concentration. Plasma protein concentration returns to normal limits within 10 to 21 days after the injection, at which time total circulating protein and plasma volume are lower than normal. Loss of protein cannot be accounted for by increase in urinary nitrogen. It is possible to maintain dogs at low levels of plasma protein concentration for several weeks by repeated injections of gum acacia solution. Anatomical changes following such a procedure in a normal dog are most conspicuous in the liver (Fig. A). These observations further implicate the liver as a source of plasma protein. Two mechanisms for the diminution of plasma protein following gum injection are suggested. One of these is based on the possibility that the liver cells being engorged with gum acacia are not able to produce the necessary amount of plasma protein to supply the normal demand. The other possibility is that with the injection of the gum, since there is obviously a greatly increased amount of colloid in the blood, the more readily removable colloid, i.e . plasma protein, is taken out of the blood stream, in an attempt to return plasma volume and colloid osmotic pressure to the normal limits. It is probable that both of these mechanisms are involved. Injection of gum acacia is suggested as a technique for further study of disturbed liver function. It must be obvious that clinical use of gum acacia for intravenous injection is not without danger.