PRIlSrcrPLES OF TRACER METHODOLOGY 27 



then 



^ A* A* 



(1-22) 



which reduces to the usual form: 



(1-23) 



Equation (1-22) is based on the fact that the removal of A* will be 

 proportional to p and to the ratio of labeled to total phospholipid. This 

 is an important concept, an understanding of which will be helpful in han- 

 dling the more complex systems. It should be noted that the constant of 

 Eq. (1-23), p/ A, which is analogous to the k of previous equations, rep- 

 resents the fraction of the phospholipids which is renewed in unit time. 

 The reciprocal A/p is the turnover time tt. The turnover time may be 

 conveniently calculated by the use of ti^:,, the time for the labeled phospho- 

 lipids to reach one-half the zero-time value. By analogy with Eq. (1-16), 



(1-24) 



(1-25) 



These relationships may perhaps be more easily visualized by the use 

 of numerical values. If there is 1000 units of phospholipids in plasma 

 which are being renewed at the rate of 500 units/hr, then it will take 

 ^^^%oo = 2 hr for complete renewal ti. If 100 units of labeled phos- 

 pholipids is introduced into the system, then the initial rate of removal of 

 the label must be 100 X ^^Kooo, which is another way of stating 

 Eq. (1-22). 



In experiments of this type it is advisable to plot log A * vs. time, since 

 any deviation from a straight line will indicate interferences that should 

 be taken into account. It is also advisable to restrict the observations 

 to a period over which a small fraction of the substance has been renewed, 

 since the calculations will not be valid when considerable numbers of 

 molecules are turned over more than once. 



Figure 1-5 presents typical data on the amount of labeled phospholipids 

 found in the blood of a dog at varying times after injection (69). The 

 P^'Mabeled phospholipids had been produced by administration of P^'- to 

 a dog, sacrifice of the animal, and isolation of the phospholipids for injec- 

 tion into another dog. It is clear that a straight-line relationship was 



