Shark immunoglobulin,

Suran and Papermaster made the remarkable observation that in five of these positions the amino acids present are also commonly found in human k chains. These studies were extended by Klaus et al. (52) who showed, first, that there are some blocked (by pyrrolidonecar-boxylic acid) as well as unblocked H and L chains in leopard shark immunoglobulins. Most of their sequence data were, however, obtained with unblocked H chains or L chains. Sequences up to position 10 showed homology of shark L chains with human k and X chains and impressive homology of shark and human H chains. Of the first 10 positions, eight residues present in the highest proportion in shark H chains are also found at the same position in some human H chains. 

Immunoglobulins are antibodies. TCR molecules are akin to antibodies.) The authors then argue that sharks, which are very distantly related to mammals, appear to have all three components. But it s one thing to say an organism has a completed, functioning system, and another to say how the system developed. The authors certainly realize this. They note that... 

A similar investigation of immunoglobulin M from horse, p% and nurse shark has been reported In both equine and porcine IgM extensive flexibflity was observed corresponding to either... 

The flexibility was more restricted in the nurse shark IgM. The above results, together with a steady-state fluorescence depolarization study on the flexibility of immunoglobulins from amphibia and reptiles has suggested that the degree of flexibility decreases with the level of phytogeny. Thus, time-resolved fluorescence depolarization measurements may provide one of the first techniques capable of indicating... 

For the IgM of the dogfish shark, Marchalonis and Edelman reported molecular weights of 980,000 (19 S) and 198,000 (7 S) (15). After reduction, alkylation, and exposure to 8 M urea, H chains (mol. wt. 72,000) and L chains (26,000) were liberated in equimolar amounts. Very similar results were obtained with the immunoglobulin of the leopard shark (33). In addition, it has been shown that the N-terminal sequence of six amino acids is the same for the H chains of the 7 S and 19 S forms (34). The N-terminal residue is glutamic acid, rather than cyclized glutamic acid this was the first of many demonstrations of an unblocked amino-terminus in an H chain. (It is now known that some of the H chains of the shark have a blocked N-terminal group.)... 

The 7 S antibodies of the dogfish shark and of the lamprey are almost unaffected by treatment with papain in neutral buffer (29). Klapper et al. investigated the effects of papain and trypsin on the 19 S and 7 S immunoglobulins of the lemon shark, and on 7 S molecules obtained by... 

Tables 7.2 and 7.3 also include partial amino acid sequences of immunoglobulins of a few other species the obvious homologies further illustrate the common evolutionary origin of the immunoglobulins. The similarity in sequence of paddlehsh H chains to those of the shark is of interest since the IgM of the paddlelish (its only class of immunoglobulin) is a tetramer, rather than a pentamer, of the 4-chain subunit and because its H chain has an unusually low molecular weight (55).

The common origin of immunoglobulins is strikingly illustrated by a highly conserved amino acid sequence (Arg or Lys-Phe-Ser-Gly-Ser-) which was detected in the Vl regions, at positions 68-72, of a wide range of species these include the nurse shark, chicken, duck, turkey, and a number of mammals (56). It is also present in mouse and human myeloma proteins only an occasional substitution is seen. The same sequence is found in k and X chains. Its structural role is, as yet, unknown. A homologous but more variable sequence is found in the H chains of several mammals lower species have not yet been examined. 

Acton et al. carried out extensive studies of carbohydrate (CHO) content and composition in the H and L chains of immunoglobulins from a variety of species. These ranged from the shark to the human and included representatives of each of the five major classes of vertebrate (103). Immunoglobulins of both high and low molecular weight were investigated. 

In contrast to IgM, IgG is capable of passing into extra vascular fluids and secretions. This characteristic may well be a function of the smaller size of the molecule, rather than its chemical structure, since 7 S shark IgM, which is antigenically identical to the 19 S shark protein, is similarly distributed in both compartments, whereas the 19 S molecule is restricted to intravascular spaces. The extravascular presence of immunoglobulins probably represents an important defense mechanism. 

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