NATURALLY OCCURRING FOLATES *

The goal pursued in the study of a family of naturally occurring compounds is threefold: one must strive 1 ) to assign an unequivocal structure to each member of the group; 2 ) to elucidate their functions; and 3 ) to ascertain their distribution in biological materials. It is obvious that the difficulties encountered in these studies will increase with the number of compounds comprising the group under investigation. Furthermore, these difficulties will be compounded if the molecules studied happen to be chemically unstable and appear in natural samples in very low concentrations. T h e study of the naturally occurring folates is fraught with all the obstacles mentioned above. The “folates” constitute a large family of natural molecules arising by reduction of, and additions to, f o l k acid (pteroylglutamic acid), the parent structure of the group.? I t is known that at least three states of reduction of the pyrazine ring can occur, that six different one-carbon substituents can be present in position N’ and/or Nl“, and that the glutamyl residue is linked in gamma peptide linkage to a poly--y-glutamyl side chain of unknown, and most likely, varied length. Even if one assumes that the polyglutamyl side chain extends to n o more than six residues, the theoretical number of folates approaches 150 (FIGURE 1 ) . This number is dramatically increased if one entertains the possibility of having more than six glutamyl residues and/or other amino acids as part of the peptide c h a i n 2 The difficulties of dealing with such a large family of related compounds are magnified when one considers their existence in low concentrations in nature, their instability, and the fact that they are found in the midst of biological samples in which enzymes capable of modifying or degrading them abound. It is therefore apparent that ou r success in reaching the goals outlined here is predicated upon the development of a rather specialized methodology. Our contribution to this monograph focuses attention o n the folylpolyglutamates as important biological compounds. Some of the problems of existing methodology will be discussed along with attempts currently under investigation in our laboratory to deal with them.