DINTZIS, BORSOOK, AND VINOGRAD 99 



i.e., one polypeptide chain. If assumption 2 above is incorrect, and only a frac- 

 tion of the microsomal particles is functional, the agreement is even better. 

 From the rate of incorporation of label into finished hemoglobin molecules and 

 the concentrations in the living cells of hemoglobin (15 per cent) and micro- 

 somes (0.5 per cent), one may calculate that to account for the production of 

 new hemoglobin each microsomal particle must make one-quarter of a hemo- 

 globin in 1.5 minutes. 



SUMMARY 



The above data lead to the picture of a microsomal particle as an almost spher- 

 ical sponge-like structure of anhydrous molecular weight 4,000,000 and diame- 

 ter 340 A. One-half of the mass is represented by ribonucleic acid which ap- 

 pears to be present as four strands of molecular weight 500,000. The half of 

 the microsomal particle which is protein appears to be almost entirely (99.9 per 

 cent) structural in nature; i.e., it is not transient protein precursor. 



Woven into this sponge-like structure in some way is a very small amount 

 (0.05 per cent by weight) of transient protein precursor. Taken together with 

 the observed rate of hemoglobin production, this amount of precursor is com- 

 patible with the conclusion that one microsomal particle makes one polypeptide 

 chain of hemoglobin in approximately 1 minute. 



Contribution No. 2338 



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