THE ACCUMULATION OF PHOSPHATE BY FERTILIZED SEA URCHIN EGGS

¹ Department of Zoology, University of California, Berkeley, California

1. The concentration of phosphate in the external medium of a suspension of unfertilized Strongylocentrotus eggs remains constant, or increases, while in a suspension of fertilized eggs, the concentration of phosphate in the external medium decreases.

2. Fertilized Strongylocentrotus eggs absorb P³² and phosphate from sea water at identical rates, revealing that the exchange of phosphate between the cell interior and the external medium is inappreciable.

3. The rate at which phosphate is removed from sea water by fertilized Strongylocentrotus eggs is relatively independent of the external concentration as long as this exceeds 15 to 20 micrograms P per liter.

4. When unfertilized and fertilized sea urchin eggs are continuously exposed to sea water containing P³² and more than 20 micrograms P per liter, 95.9 to 96.4 per cent of the P³² which enters the eggs is found in the trichloroacetic acid-soluble fraction, with 3.6 to 4.1 per cent of the P³² being recovered in the acid-insoluble fraction. The distribution of P³² between these two fractions is not significantly different in the unfertilized, as compared to the fertilized eggs. Although a slightly lower proportion of P³² was found in the acid-insoluble residue of unfertilized eggs, outward leaching of P³² during the washing of the unfertilized eggs may well account for the difference noted.

5. If fertilized Lytechinus pictus eggs containing P³² are suspended in a non-radioactive medium shortly after insemination, the proportion of P³² in the acid-insoluble fraction increases from 3.6 per cent at the two-celled stage to 7.0 per cent at the blastula stage.

6. The concentration of inorganic P in the trichloroacetic acid-soluble extracts of the eggs decreases prominently following insemination. A corresponding increase occurs in the quantity of P liberated after 10 minutes' hydrolysis of the extracts in 1 N HCl at 100° C.

7. The major portion of the P³² which enters the eggs is found in the easily hydrolyzable P fraction of the trichloroacetic acid-soluble extracts. After fertilization, the proportion of P³² in the easily hydrolyzable P fraction increases.