Functions of Invertebrate Hemoglobins with Special Reference to Adaptations to Environmental Hypoxia

The oxygen-binding properties and some associated structuralfeatures of invertebrate hemoglobins (Hb) are reviewed togetherwith the environmental conditions (particularly in stressfulhabitats) that influence oxygen-binding functions in nature.Compared to the vertebrates invertebrate Hbs display a spectacularvariation in functional properties, which is manifest withinthe major systematic groups of animals and within the differenthistological and molecular categories of the pigment. This variabilityis seen primarily to represent adaptations to the wide rangeof environmental conditions to which Hb-bearing invertebratesare subjected in nature but also to reflect certain organismicfunctions, which determine the internal conditions under whichthe Hbs work in life. These adaptations appear to be uniquelydirected towards optimizing the transfer of oxygen from theenvironmental source to the mitochondrial combustion sites.The immense structural and functional complexity of invertebrateHb systems may well compensate for a decreased organizationat the organ level compared to that in vertebrates shiftingmore of the regulatory burden to the molecular level.     

Divergence of the Two Albumins of X. laevis

The mRNAs coding for the 68,000 and 74,000 dalton serum albumins of Xenopus laevis were purified by hybridisation to their corresponding cloned cDNA and translated using the reticulocyte lysate. The primary translational product of the 68,000 dalton albumin has a molecular weight of 70,000 daltons suggesting that it is synthesised with a signal peptide which is cleaved during secretion. In contrast, the primary translational product of the 74,000 dalton albumin has a molecular weight of 72,000 daltons suggesting that it must be posttranslationally modified to account for the increased molecular weight of the mature protein. X. laevis oocytes injected with albumin mRNA secrete proteins of the same molecular weights as the mature albumins. When these translational products were chromatographed on concanavalin A Sepharose, the 74,000 dalton albumin was bound suggesting that it is glycosylated. Comparison of X. laevis and X. tropicalis albumins suggests that the 68,000 dalton albumin is similar to the primitive Xenopus albumin and that since the genome duplication which occurred in X. laevis , differences have arisen in both the length and processing of the primary translational product to account for the current difference in the molecular weights of the two X. laevis albumins.     

DNA SYNTHESIS AND CELL TURNOVER IN RANA PIPIENS TADPOLE LIVER DURING SPONTANEOUS METAMORPHOSIS1

The relationship of DNA synthesis and cellular turnover to biochemical differentiation during metamorphosis of R. pipiens liver was investigated. Average DNA/cell was constant at 11.6 pg/ nucleus through stage XXV; but increased during juvenile growth; during metamorphosis stages, changes in total DNA content must correspond to changes in cell number. Rates of DNA synthesis were estimated by rates of ³H-thymidine incorporated into the acid-precipitable fractions, corrected for both precursor uptake into the acid-soluble pool, and for endogenous thymine pool size. DNA content increased steadily from premetamorphosis until late prometamorphosis; at preclimax stages XVIII and XX there were two successive decreases in DNA content of approximately 30%. Fluctuations in synthesis rates preceded corresponding fluctuations in content; DNA synthesis was maximal at stages XVI and XVIII, decreased nearly ten-fold at metamorphic climax, and then gradually rose again during late climax stages. The size of the endogenous thymine pool increased transitorily during spontaneous metamorphosis corresponding to a stage of maximal DNA synthesis. These results indicate that both DNA synthesis and cellular turnover play a significant role in determining net DNA synthesis rates and content during metamorphosis. Metamorphosis of the tadpole liver appears to be associated with both proliferation and cellular death, perhaps a replacement of “larval” by “adult” cells. Metamorphosis of the liver cannot be occuring in a “fixed population of cells” as is commonly assumed. An interpretation of the population dynamics of the metamorphic liver is presented.