PROGRESS IN PROTOZOOLOGY

260 D. L. NANNEY
Esterase-1 is a propionyl esterase, activated by sodium taurocholate and
inhibited by eserine sulfate. Esterase-2 is a butyryl esterase activated by
p-chloromercuribenzoate and insensitive to eserine. Finally esterase-3
acts on both a-naphthyl propionate and butyrate; it is activated by both
sodium taurocholate and p-chloromercuribenzoate and is insensitive to
eserine. These specifications seemed suitable for identifying the arrays
of isozymes in the various species.
However, the esterases differ not only in their electrophoretic mobilities
but also in their other enzymatic characteristics. Three species
manifested isozymes of all three classes, but in the other species one or
more of the classes is missing entirely. T. pigmentosa and T. hyperangularis
have no enzymes that can be unequivocally associated with
esterases-1, -2, or -3. These observations do not prove that homologous
proteins are missing from the other species, or even that the homologous
proteins are no longer esterases. They do demonstrate that some of the
functional characteristics of these enzymes are labile within the complex,
despite the superficial physiological and nutritional similarities of the
organisms.
Simply to show that the variability of the esterases is no isolated
example, consider for a moment the results of a recently published
(Nanney et al. 1981) starch gel electrophoretic analysis of the NADPisocitrate
dehydrogenase enzymes. T. pigmentosa (Table 11) manifests
three isozymes — a proximal, a middle and a distal band. Other species
show one, two, or three bands, and their positions within the general
region in which they appear are variable. We can arbitrarily classify
a species as to the isozymes present (or absent) within each region of
the gel, and as to their relative mobility (fast or slow). Although this
procedure provides "characters" that can be used in sorting the species,
we cannot be confident of the genetic relationships of the molecules
being compared. This enzyme may be somewhat more evolutionarily
constrained than are the esterases, but not much more. The enzyme's
lability is too great for it to be used as a reliable measure of evolutionary
distance in species so distantly removed from a common ancestor. This
same conclusion is probably appropriate for isozymic studies on several
other enzymes (Borden et al. 1977); in many cases one simply cannot
compare the electrophoretic mobility of enzymes for two species, because
the enzymes are too different to be visualized under the same circumstances
on a gel. Such data may be helpful for taxonomic purposes, in
the identification of a species; they are probably useless in defining
phylogenetic relationships.
Primarily because the form of Tetrahymena is so perfectly preserved
in all the species of the complex, one might suppose that structural
proteins would be more conservative in this group than are many of the
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