Ds. Auld et al., X-RAY-ABSORPTION FINE-STRUCTURE AS A MONITOR OF ZINC COORDINATION SITES DURING OOGENESIS OF XENOPUS-LAEVIS, Proceedings of the National Academy of Sciences of the United Statesof America, 93(8), 1996, pp. 3227-3231
The x-ray absorption fine structure (XAFS) zinc K-edge steps for intac
t stages I,II and V,VI Xenopus laevis oocytes demonstrate that the zin
c concentration is about 3 and 1 mM, respectively, However, the chi(k)
function for the early stage oocytes differs markedly from that for t
he late one, Analysis of the XAFS data for stage I,II oocytes indicate
s that zinc is bound to 2.0 +/- 0.5 sulfur atoms at an average coordin
ation distance of 2.29 +/- 0.02 Angstrom and 2.0 +/- 0.5 nitrogen or o
xygen (N/O) atoms at 2.02 +/- 0.02 Angstrom, In marked contrast, in st
age V,VI oocytes, zinc is bound to 4.1 +/- 0.4 N/O atoms at an average
distance of 1.98 +/- 0.01 Angstrom. Our previous studies demonstrated
that 90% of the zinc in stage VI oocytes is sequestered within yolk p
latelets, associated with a single molecule, lipovitellin, the proteol
ytically processed product of vitellogenin. XAFS analysis of yolk plat
elets, lipovitellin, and vitellogenin demonstrates that zinc is bound
to 4.0 +/- 0.5 N/O ligands at an average distance of 1.98 +/- 0.01 Ang
strom in each case, identical to that of stage V,VI oocytes, The highe
r shell contributions in the Fourier transforms indicate that two of t
he N/O zinc ligands are His in both stage V,VI and I,II oocytes, The r
esults show that in stage I,II oocytes, there is a high concentration
of a zinc protein whose zinc coordination site likely is composed of (
His)(2)(Cys)(2), such as, e,g,, TFIIIA. As the oocytes develop, the pr
edominant zinc species becomes one that exhibits the (His)(2)(N/O)(2)
zinc site found in lipovitellin, Hence, the ligands to the zinc atoms
in intact oocytes and the changes that take place as a function of oog
enesis and after their fertilization, during embryogenesis, now can be
examined and explored.