Evidence against the occurrence of artifacts due to carrier ampholyte-protein binding during isoelectric focusing

Ionic binding properties of carrier ampholytes

Radioactive ampholytes were synthesized with specific activity of 638 microCi/g. These were used in studies of ampholyte binding to target proteins under non-ionic conditions. These radioactive ampholytes bound to target proteins but were dissociable in sodium chloride solutions with dissociation occurring in a concentration dependent way. The ampholytes could be dissociated from target molecules using excess unlabelled ampholytes synthesized in the laboratory as well as commercial ampholytes. Radioactive ampholytes were bound to target proteins with different isoelectric points and the bound ampholytes were eluted and analyzed by recycling isoelectric focusing. The results showed that acidic proteins bound basic ampholytes and basic proteins bound acidic ampholytes. Acidic radioactive ampholytes were selectively bound by Sephacryl S-200 and ampholyte exchange from protein to Sephacryl S-200 was shown.

Binding of polyanions to carrier ampholytes in isoelectric focusing

The binding of carrier ampholytes to polyanions is markedly pH-dependent: it is very strong at pH 3, rather weak at pH 5 and abolished at pH 7. Binding is affected by the type of negative charge, its density and spatial orientation on the polyanion. On the basis of the type of negative charge, the binding strength decreases in the following order: polyphosphate greater than polysulphate greater than polycarboxylate. Given the same type of negative charge, the binding is dependent on charge density and its space orientation: thus polyglutamic acid forms stronger complexes than polygalacturonic acid. The minimum length of the polyanion eliciting a measurable binding appears to be of the order of about six negative charges, as demonstrated with hexametaphosphate.

Biological and immunological characterization of human luteinizing hormone: I. Biological profile in pituitary and plasma samples after electrofocusing

Pituitary and plasma pools from postmenopausal women and plasma pools from women at midcycle were fractionated by electrofocusing in sucrose density gradients. The biological LH activity was determined in each of the electrofocusing fractions by the use of an in vitro bioassay method. A heterogeneous profile of LH activity was found in both pituitary and plasma samples with a large proportion present within the pH range 6.5-10. In a total of 11 electrofocusing runs 7 main regions of high LH activity were found within this range with mean pI values (+/- SD) of 6.75 +/- 0.08 (n = 6), 7.33 +/- 0.08 (11), 7.80 +/- 0.09 (11), 8.23 +/- 0.10 (11), 8.81 +/- 0.04 (7), 9.17 +/- 0.05 (6) and 9.55 (2). A significantly higher proportion of LH activity was found in the midcycle plasma samples (36%) in the pH regions with mean pI values of 8.81, 9.17 and 9.55 than in postmenopausal plasma (7%) and pituitary extracts (5%). This indicates that the profile of biologically active LH in women in the fertile age is different from that present in postmenopausal women. By detailed fractionation based on narrow pH range studies and the refocusing of specific peak fractions it was shown that each of the regions studied consisted of several peaks of LH activity indicating the presence of a large number of molecular species exhibiting varying degrees of LH activity. The relative proportions of these species showed considerable differences between sources but also between samples from the same source.

Heterogeneity of storage proteins in maize

The extensive charge heterogeneity of maize (Zea mays L.) zeins observed in isoelectric focusing (IEF) (about 15 bands with pI's in the pH range 6-9) has been found to be independent of extraction procedures or of endosperm development. Zeins do not stain for glycoproteins and exhibit only one lipoprotein component, with pI 3, representing 3-5% of the total protein.Zeins are very resistant to in vitro deamidation, at both acidic and alkaline pH, at high temperatures, and for rather prolonged times. On the basis of the zein content in acidic and basic amino acids, and of the respective pI's exhibited in IEF (mostly in the pH range 7-8) it has been calculated that at least 90% of the glutamic and aspartic acids (≈52 residues out of a total of ≈ 190) are present as asparagine and glutamine.Amino acid analysis of zein fractions isolated by preparative IEF has demonstrated changes in the composition of 18 amino acid residues. However, since these changes affect only neutral and hydrophobic residues, it is concluded that the observed zein heterogeneity is partly based on in vivo deamidation of glutamine and asparagine and partly to spot mutations in some of the genes responsible for zein synthesis.