Nonuniform distribution of enzymes in fish eggs

The distribution of enzymes in the blastoderm and the yolk was investigated in the eggs of the bony fish, Misgurnus fossilis. The total activities of lactate dehydrogenase (LDH), glucose-6-phosphate dehydrogenase (G-6-PD), and aldolase (ALD) did not change during early development. However, the enzymes were redistributed in the egg: at the blastula stage the blastoderm contains about 60% of the total ALD activity and 80% of the total LDH and G-6-PD activities. The "free cytoplasm" volume, in which the enzymic proteins can be solubilized, is greater in the yolk than in the blastoderm. Low levels of homologous enzymes from other species injected into the egg are distributed between the blastoderm and the yolk in the same way as are the native enzymes; they are primarily localized in the blastoderm. Injected exogenous enzyme concentrates preferentially in the yolk, apparently being distributed proportionately with respect to the "free cytoplasm" volumes. The ratio of these "free cytoplasmic" volumes, determined by distribution of the injected enzyme (LDH), in the yolk and in the blastoderm is 6.8. On the basis of our results we propose that many egg enzymes are noncovalently bound to blastoderm structures, and that the "nonbound" fraction of the enzymes in the cytosol is distributed proportionally to the "free cytoplasm" volumes in the blastoderm and in the yolk. Fractionation of the subcellular constituents of blastoderm extracts by ultracentrifugation or by gel filtration on Sepharose in the presence of an excess of LDH reveals the high-molecular weight LDH-containing complexes. These complexes are absent in the yolk and in the unfertilized eggs. These results are consistent with the hypothesis that the binding of the enzyme to subcellular structures in the blastoderm is an important mechanism in ooplasmic segregation.