84 A. Xeuberger 



quickly into the carbonic anhydrase. If the labelHng is done bio- 

 logically over a longer period, however, the radio-zinc will go into the 

 carbonic anhydrase. It may be possible, using ^^Zn, to get something 

 for comparison with haemoglobin; that is, if you feed the zinc to a rabbit 

 and separate the carbonic anhydrase from the red cells you may get 

 something which is very near your haemoglobin value. You will, 

 however, have to exclude other zinc compounds, from which the zinc 

 can be dissociated fairly readily. They may be zinc proteins. I under- 

 stand from Dr. B. L. Vallee that he has recently obtained a new zinc 

 protein from human leucocytes, and my colleagues and I believe there 

 may be certain other zinc compounds besides carbonic anhydrase in the 

 red cell. That is one of the points you will have to consider if you use 

 6 5Zn for labelling red blood cells. 



I believe you are still worried about that 15 per cent of activity 

 which remains in the haemoglobin after about 200 days. It might 

 help to solve that problem if you injected ^^N-labelled haemoglobin. 

 Then you might get some idea of how that haemoglobin is used. 



Neuberger: I think one obvious way of investigating this is to take 

 red cells from one human being, label them and then transfuse them 

 into another experimental subject and see how the isotope content 

 decays. We w'ould have to use a large amount of glycine and it would 

 be rather expensive. 



Rittenberg: The curve obtained by plotting the ^^N content of the 

 haem against time in normal man, which Dr. Neuberger showed at the 

 beginning of his paper, has attracted our attention for quite a time. 

 There are certain difficulties about this curve. The curve does not 

 approach zero sufficiently rapidly. It is not too clear whether we are 

 dealing with a small re-utilization of the haemoglobin, or w-hether the 

 zero point for the ^^N curve is raised. In other words, you may have 

 changed the "normal abundance" in this subject so that to obtain 

 atom per cent excess you should subtract from the absolute ^^N con- 

 centration a value greater than 0-368 per cent ^^N. I think the plasma 

 protein labelling at this time was somewhat less than the haemoglobin, 

 being about • 060 atom per cent excess. Although of course there may 

 be reutilization, I don't believe it is really very great. Dr. London is 

 now studying the question that Dr. Wormall just raised, the utilization 

 of haemoglobin as such. 



As far as the middle portion of the curve is concerned, interpretation 

 is difficult because there is nothing you can do about bringing the level 

 of the precursor down rapidly. What you really ought to do is work with 

 a precursor whose concentration went up sharply and then rapidly 

 declined to zero. Since this has not been done you have to go through 

 some complicated arithmetic. The "plateau" certainly goes down, and 

 this downward trend may be due in part to a little extra synthesis or com- 

 pletion of haemoglobin synthesis in the circulating red cells. We feel that 

 this is rather small. The major cause for the downward slope of the 

 plateau is probably a small random destruction of the circulating red cells. 



WTien we studied sickle cell anaemia and plotted the data semi- 

 exponentially, we got a curve that seemed to be the sum of two straight 



