Are Yolk Phosvitins Carriers for Specific Cations? Comparative Microanalysis in Vertebrate Yolk Platelets

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Abstract
Qualitative X-ray microanalysis of frozen-dried cyclo-stome, teleost and amphibian yolk platelets has demon­ strated the general and undoubtable presence of P, S, Cl (!) and K, the probable presence of Na and the irregular presence of Ca and Mg. Iron occurred only in trace amounts if at all. Details of the process of yolk utilization by the embryo are still enigmatic. Indirect evidence ob­ tained from particular features of the glycolipophos-phoprotein molecules building up the yolk platelets as main constituents has, therefore, become a matter of recent interest. Thus, the highly phosphorylated phosvitins ("cation exchange-resin") may play a role as carrier of cations essential for embryonic growth (Taborsky [1]); studies on cation-binding as well as on the entailing conformational changes of the phos­ vitin molecule are numerous (Taborsky [1, 2]). Re­ cent observations on yolk platelets in this laboratory, however, do not fit into the frame of a very specific role of yolk-phosphoproteins in the transport of selected cations. Although the investigation has been performed only on the usual qualitative basis, the broad collection of material and the regularity of findings render the observations significant. Cryosections of whole eggs (Lampetra planeri [Bloch], Cyclostomata; Pelvicachromis pulcher, Cich-lidae, Teleostei; Triturus sp., Salamandridae, Am­ phibia) or isolated yolk platelets (M yxine glutinosa L., Cyclostomata) were dried at subzero temperatu­ res onto pure carbon specimen supports for scanning electron microscopy. Lamprey and teleost material fixed in Na-phosphate buffered glutaraldehyde was used for comparison. Elemental analysis was carried out at 25 kV in a Philips PSEM 500 scanning elec­ Reprint requests to Prof. Dr. R. H. Lange. 0341-0382/81/0700-0686 $01.00/0 tron microscope equipped with energy-dispersive Edax system (3 -4 spectra -100000 to 400000 total counts each — per species, point probe diameter 1 |im). Ooplasmic inclusions corresponding both in size and frequency to yolk platelets of the various species were identified in the scanning mode and a point probe was then positioned on their interior. Ele­ ments markedly and undoubtedly present in all un­ fixed samples and absent in the carbon support be­ tween single platelets were: P, S, Cl, and K. Ca was also present in Lampetra, Pelvicachromis and Triturus (as judged from its clear K β peak at 4.01 keV, while Ca-K or at 3.69 keV fuses with K-K β at 3.59 keV). Due to the peculiar background spectrum (Lange and Blödom [3]) we shall not enter into detailed dis­ cussion of the probable presence of some Na and Mg. Fe was never unequivocally demonstrated, but traces might have occurred in the cyclostomes. Whereas P and S probably represent covalently bound protein constituents, the presence of chloride (only irregularly present following fixation, abso­ lutely absent from neutral underground) in this highly anionic protein ambiance is remarkable. Re­ sults with respect to iron were disappointing, espe­ cially considering recent biochemical work (Tabors­ ky [2]). It is concluded that K+ is regularly and undoubt­ edly present in considerable amounts in yolk pla­ telets; this holds true for Ca2+, but to a lesser degree (not found in M yxine). The occurrence of Na+ (and in Lampetra some Mg2+) is probable but evidence is at present unsatisfactory for technical reasons. The presence of traces of Fe cannot be excluded. Due to the presence of chloride, the above cations can only in part be bound by the phosphate groups of the highly anionic yolk-platelet proteins. By combining the recent results in yolk-platelet crystal research (Lange [4]; Ohlendorf et al. [5]) with morphometry of the ooplasm and attempts at micro-analytic quantitation, important quantitative data on both a relative and absolute scale will eventually become available.

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