A novel protease may explain widely differing models for the structure of Artemia salina haemoglobin

Physical, biochemical and functional characterization of haemoglobin from three strains of Artemia

The brine shrimp, Artemia, an inhabitant of coastal and inland salterns, encounter fluctuations in the salinity which in turn influences the oxygen availability of their habitat. Hence, experiments were performed to analyze variations in haemoglobin structure and patterns of three strains of Artemia from South India and also to reflect the effect of varying oxygen levels in their habitat. Haemoglobins were purified on a DEAE-Sephadex column and haemoglobin types were analyzed by comparing their relative mobility on a non-denaturing medium. Furthermore, their molecular masses were determined by gel filtration in Sepharose column and by dodecylsulfate polyacrylamide gel electrophoresis. Results clearly reveal the presence of three distinct extracellular haemoglobins Hb I, Hb II and Hb III in Tuticorin strain while the other strains displayed only trails or the complete absence of Hb III and Hb II. Estimated molecular masses of these haemoglobins are 235,000-250,000 Da. Denaturation of the reduced and alkylated haemoglobins revealed apparently one polypeptide chain with a molecular mass of 124,000 Da. Upon denaturing gel electrophoresis of native haemoglobin Hb II, it was found that the 124,000 Da, polypeptide was cleaved specifically into two unequally-sized fragments of 50,400 and 79,800 Da. With regard to oxygen affinity, Hb III has a very high affinity for oxygen, an almost negligible Bohr effect and a good physiological adaptation to temperature changes. By combining the three haemoglobins in different proportions Artemia strains must be able to withstand diverging environmental conditions. In particular, the absence of Hb III in Puthalam and its occurrence as a faint band in Thamaraikulam could be correlated to the oxygen levels of their habitats.

Structure of extracellular hemoglobin from the brine shrimp Artemia salina

1. Hemoglobin from the brine shrimp Artemia salina, purified by ultracentrifugation and preparative gel electrophoresis in non-denaturing medium, gave in sodium dodecyl sulfate-polyacrylamide gel electrophoresis a single band corresponding to a polypeptide chain with Mr 150,000. 2. Crosslinking by glutardialdehyde resulted in the appearance of a band corresponding to a molecular mass twice that of a polypeptide chain. 3. Limited trypsinolysis gave eight proteolytic bands corresponding to submultiples 8/9-1/9 of a polypeptide chain. 4. We conclude that a molecule of Artemia hemoglobin is composed of two single polypeptide chain subunits and that each subunit consists of nine structural units roughly equal in size.

Multiple molecular forms of rat mammary glucose-6-phosphate dehydrogenase: proposed role in turnover of the enzyme

Polyacrylamide gel electrophoresis shows that more than 80% of the total glucose-6-phosphate dehydrogenase activity of rat mammary homogenates exists as three dimeric forms. During lactation all three forms are present in equivalent amounts, but 24 h after the removal of the pups to stop lactation, there is a shift to the fastest migrating form (band I). During this period the ratio of enzyme activity to tissue glutathione concentration decreased. Using partially purified preparations of the enzyme it was shown that dithiothreitol could convert all the activity to the slowest migrating form (band III) and that diamide could reverse this change giving rise to band I only. Reduced glutathione could partially convert the enzyme to the band III form. Experiments using iodoacetic acid, iodoacetamide, p-chloromercuribenzoate, and mercuric chloride are also described. Based on these observations it is proposed that band I and band III represent fully oxidized and fully reduced forms of the enzyme, respectively, and band II a partially oxidized form. The oxidized and reduced forms are catalytically equivalent. However, the oxidized form is the most rapidly inactivated by chymotrypsin or mammary microsomes. Mammary microsomes can also catalyze the oxidation of the enzyme. The oxidation precedes the inactivation by microsomes and also occurs at lower microsome concentrations. It is proposed that the microsomal oxidation of the enzyme is the initial step in the turnover of the enzyme.

Artemia hemoglobins. Increase in net synthesis of the beta-polypeptide (relative to the alpha-polypeptide) in hypoxia

Previous studies have shown that in the brine shrimp there are three dimeric hemoglobins with polypeptide composition alpha 2, alpha beta, beta 2. Concentrations of the alpha- and beta-polypeptides increase in hypoxia. We now report a two-dimensional electrophoretic method for assay of radiolabelled polypeptides in each hemoglobin. Net synthesis (synthesis minus degradation) of the beta-chain, relative to that of the alpha-chain, increases more than 3-fold (in male and female adults) within 3 days following a downshift in oxygen concentration from 0.2 to 0.1 mM in the culture medium. 3 days after downshift (2 days after in vivo incorporation of radiolabelled leucine), the beta-homodimer contained 10-20% of the radiolabel in the three hemoglobins although beta 2 was usually not detectable in the protein stain of an overloaded gel. The amount of radioactive leucine incorporated per unit amount of protein was more than 300-times greater in the beta 2 homodimer than in the beta-subunit of the heterodimer, suggesting that beta 2 does not dissociate rapidly during electrophoresis on the first dimension non-denaturing gel. This evidence for stable association of the two beta-monomers and the 5-8 heme-binding domains within each monomer (in vivo and during electrophoresis on non-denaturing gels) allows us to exclude one of two alternative interpretations of genetic data published previously. We present an independent line of evidence for the dimer model of the native hemoglobins (which states that each polypeptide has many heme-binding domains).