Curtis A. Williams

Antigenic Correspondence of Serum Albumins among the Primates

The relative cross-reactivities as judged by tile quantitative precipitation reaction of 22 primate serum albumins were determined with pooled antiserums against human Serum albumnini. The atitgenic correspondences to human serum albumin of all but three serum albumin fell into narrow ranges, and the groups defined by these ranges are distinct taxonomic categories. The values of the cross-reactivity ranges are consistent with the presumed phylogenetic relationships to man on the assumption that the evolutionary modification of protein structure has been progressive and divergent.

Experimental and evolutionary significance of similarities among serum protein antigens of man and the lower primates.

The remarkable similarities observed in the protein complement of blood sera of mammals and the intimate relationship of its constituents to physiology, both normal and abnormal, have made its investigation one of the most engaging of biochemical research. The more intensive the investigation, however, and the more discriminating the tools of the investigator, the more striking and meaningful do the differences appear to become. Immunochemicdl techniques such as immunoelectrophoresis are proving extremely useful when a problem requires the detection of similarities and differences among complex mixtures of macromolecules. Among such mixtures, human serum proteins in their normal and pathological expressicn, have received by far the most a t t e n t i ~ n l ~ since the introduction of the immunoelectrophoretic technique.6 If the enormous fund of information about this mixture of proteins could be directly transferable to closely related species, research involving serum proteins in these animals would be significantly advanced. The use of antisera against the antigens of one species or strain to examine the antigens of another is not a new concept. Some systems of classification of bacterial strains are based upon the relative specificity of heterologous reactions, but this is only one of the applications of systematic serology. Very few studies, however, have employed immunoelectrophoresis to compare antigenic relationships among living forms, and those that h a ~ e , ~ , ~ with the exception of one: were not devoted to several members of closely related systematic groups. The general plan of this study was to obtain immunoelectrophoretic patterns for a number of primate species, representing the three superfamilies of the suborder Anthropoidea. Although possible applications of this approach to experimental medicine and biology are pointed out, our primary object was to assess the observations and data in terms of primate evolution and systematics.

DISCUSSION OF BIOCHEMICAL AND IMMUNOCHEMICAL APPROACHES TO THE STUDY OF PROTEIN EVOLUTION IN THE PRIMATES

The study of proteins as a window into the evolutionary process perhaps has been elevated from a dark glass to a rose-colored glass with the advent of so many analytical techniques now at our disposal. It takes a considerable investment of time and energy to adapt methods to new purposes, and evolutionists are frequently loath to abandon them without forcing the results into some evolutionary theory or hypothesis. Nevertheless, our ambitions are well motivated and scientifically sound. It is accepted generally that primary gene function is to direct the synthesis of polypeptide chains which make up proteins. Deletions, insertions, and substitutions of information in a genetic code constitute mutations that are the starting points for evolutionary change. The prospects of this knowledge are enormous for evaluating known experimental or natural modifications in relation to properties or function of proteins. With enzymes and specialized cellular proteins such as hemoglobin, much can be learned. Chemical and physical analyses can give us information on the effects of changes in covalent structure ( peptide and disulfide linkages ) , and eventually, perhaps, in the three-dimensional conformation. Where modifications are not calamitous, however, but rather minor and cumulative during the course of evolution, investigation of the general properties of the intact, active proteins might be more illuminating to students of evolution than the demonstration of specific chemical modifications of the covalent structure. Direct physico-chemical techniques may detect differences among homologues from different species, and chemical analyses may determine the exact nature of the differences; but these data are difficult at best to interpret in terms of evolutionary trends, particularly if function is not impaired or detectably modified. Immunochemistry is an indirect approach to structural change in protein structure. While most immunochemical analyses are not likely to identify a particular mutation, they may detect it, and they will certainly detect a critical accumulation of mutations. Since evolution in higher organisms is considered to be characterized more by accumulation of minor changes than by the chance successes of single major modifications, it would seem that immunochemistry would prove very promising for the study of protein structure in animal relationships. It must be remembered, however, that the immunochemist is dealing primarily with surface conformations of large complex molecules. While this level of structure reflects, and is at least in part determined, by the peptide sequence and the

DISCUSSION PAPER: INTERPRETATION OF IMMUNOCHEMICAL CORRESPONDENCE OF PRIMATE PROTEIN HOMOLOGUES

The recent rapid advances in our understanding of the genetic control of protein structure and the consequent illumination of evolutionary changes at the molecular level have been reviewed by several authorsl4 and have been the topic of several symposia.5-8 Lichter has reviewed the use of immunological metliods to assess evolutionary change (this monograph). This paper takes its theme from a discussion of Dr. Lichter’s review. We have included no new data; rather, we have attempted to reassemble previously published material to illustrate points of interpretation that seem of utmost importance to us. Moreover, many of those points are restatements taken from a more extensive reviewg or an earlier discussion written in similar circumstances.*O The important questions posed by the comparative analysis of protein homologues are: 1) What do relative amounts of similarity (or difference) tell us about evolution in general? 2) What do specific modifications reveal about function and selection at the molecular level? 3) Can selection of molecular modifications and natural selection of organisms be equated? Clearly, we cannot yet answer these questions, but we can take positions in order to design or interpret experiments. The articles of Simpson,I1 Margoliash and Smith,12 and Zuckerkandl and Pauling13 are particularly thought-provoking, and are recommended as illustrations of the range of views on these matters.