C-terminal Basic Amino Acids Research Articles

Absence of procarboxypeptidase R induces complement-mediated lethal inflammation in lipopolysaccharide-primed mice.

Carboxypeptidase R (CPR) is a heat-labile enzyme found in serum in addition to stable carboxypeptidase N. CPR cleaves the C-terminal basic amino acids, arginine and lysine, from inflammatory peptides such as complement C3a and C5a, bradykinin, and enkephalin. This enzyme is generated from procarboxypeptidase R (proCPR), also known as thrombin-activatable fibrinolysis inhibitor, following cleavage by proteolytic enzymes such as thrombin, plasmin, and trypsin. We generated proCPR-deficient mice by knocking out exons 4 and 5 of the proCPR gene, which are regarded as essential for CPR function. At LPS challenge, there was virtually no difference in lethality among proCPR(+/+), proCPR(+/-), and proCPR(-/-) mice. However, challenge with cobra venom factor, which can activate and deplete almost all complement in vivo, induced a lethal effect on proCPR(-/-) mice following LPS sensitization which up-regulates C5a receptor expression. In contrast, proCPR(+/+) and proCPR(+/-) mice were able to tolerate the cobra venom factor challenge with the limited dose (30 U). Although carboxypeptidase N plays a role in inactivation of inflammatory peptides in vivo, CPR may also be important in the regulation of hyperinflammation.

A C-terminal basic amino acid motif of Zaire ebolavirus VP35 is essential for type I interferon antagonism and displays high identity with the RNA-binding domain of another interferon antagonist, the NS1 protein of influenza A virus

The ebolavirus VP35 protein antagonizes the cellular type I interferon response by blocking phosphorylation of IRF-3, a transcription factor that turns on the expression of a large number of antiviral genes. To identify the domain of VP35 responsible for interferon antagonism, we generated mutations within the VP35 gene and found that a C-terminal basic amino acid motif is required for inhibition of ISG56 reporter gene expression as well as IFN-β production. Remarkably, this basic amino acid motif displayed high sequence identity with part of the N-terminal RNA-binding domain of another interferon-antagonist, the NS1 protein of influenza A virus.

Tumor promoter binding of the protein kinase C C1 homology domain peptides of RasGRPs, chimaerins, and Unc13s

Recent investigations discovered nonkinase-type phorbol ester receptors, RasGRPs, chimaerins, and Unc13s. Phorbol ester binding occurs at the cysteine-rich sequences of about 50 residues in the C1 domains of these receptors. Fifty-one-residue RasGRP C1 peptides except for RasGRP2 showed significant phorbol 12,13-dibutyrate (PDBu) binding, but the K d values of the RasGRP1 and RasGRP3 C1 peptides were about 10-fold larger than those for the corresponding whole enzymes. Addition of the C-terminal basic amino acid cluster decreased their K d values about 10-fold, suggesting that the positive charges of these C1 peptides play an important role in the PDBu binding in the presence of negatively-charged phosphatidylserine. The 51-mer chimaerin C1 peptides showed potent PDBu binding, while the Unc13 and Munc13-1 C1 peptides without sufficient positive charges hardly bound PDBu. By the rapid screening system using this C1 peptide library, 5-prenyl-indolactam-V was identified as a promising lead for the novel protein kinase C isozyme specific ligands.

Deficits in reproduction and pro-gonadotropin-releasing hormone processing in male Cpefat mice.

Cpe(fat/fat) mice are obese, diabetic, and infertile. These animals have a point mutation in carboxypeptidase E (CPE), an exopeptidase that removes C-terminal basic amino acids from peptide intermediates. The mutation renders the enzyme unstable, and it is rapidly degraded. Although the infertility of Cpe(fat/fat) mice has not been systematically investigated, it is thought to be due to a deficit in GnRH processing. We have evaluated this hypothesis and found hypothalamic GnRH levels to be reduced by 65-78% and concentrations of pro-GnRH and C-terminal-extended intermediates to be high. Basal serum gonadotropin contents are similar among wild-type, heterozygous, and homozygous mice. Testis morphology and function are abnormal in older obese Cpe(fat/fat) mice. Matings between homozygous mutants yield a 5% pregnancy rate. By comparison, when 50-d-old Cpe(fat/fat) males are paired with heterozygous females, rates increase to 43%, and they rapidly decrease to negligible levels by 120 d. As fertility declines without accompanying changes in the hypothalamic-pituitary-gonadal axis and before obesity is evident, reproduction is more complex than originally thought. This suspicion is confirmed in 90-d-old Cpe(fat/fat) males, who readily interact with females, but rarely mount and fail to show intromission or ejaculation behaviors. Together, these findings show that CPE is a key enzyme for pro-GnRH processing in vivo; however, the reproductive deficits in Cpe(fat/fat) males appear to be due primarily to abnormal sexual behavior.

Quantitation of neuropeptides in Cpe(fat)/Cpe(fat) mice using differential isotopic tags and mass spectrometry.

Neuroendocrine peptides play important roles as intercellular messengers. We previously developed a technique to isolate and identify a large number of neuroendocrine peptides from Cpe(fat)/Cpe(fat) mice (Che, F.; et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 9971-6); these mice lack carboxypeptidase E activity and this defect causes an accumulation of neuropeptide intermediates that contain C-terminal Lys or Arg residues (Naggert, J. K.; et al. Nat. Genet. 1995, 10, 135-42). In the present study, we have developed a differential isotopic-labeling technique that can be used to quantitate changes in neuropeptide levels in Cpe(fat)/Cpe(fat) mouse tissues. Samples are treated with either the H6 or the D6 form of acetic anhydride, peptides that contain C-terminal basic amino acids are isolated by affinity chromatography on anhydrotrypsin agarose, and the isolated peptides are analyzed by mass spectrometry. Measurement of the regulation of pituitary peptides in response to dehydration showed a decrease in vasopressin. The general method described in this report should be widely applicable to a large number of neuroendocrine peptides, known and novel, in a variety of regulatory paradigms.

Peptide synthesis in organic media with subtilisin 72 immobilized on poly(vinyl alcohol)-cryogel carrier

Serine proteinase subtilisin 72 was covalently attached to the beads of poly(vinyl alcohol)-cryogel, a macroporous hydrogel prepared by the freeze–thaw technique. The immobilized enzyme was examined as a catalyst in the synthesis of protected peptides Z-Ala-Ala-Xaa-Phe-pNA (Xaa=Leu, Glu, Lys) in acetonitrile/dimethylformamide mixtures. Immobilized subtilisin catalyzed with high yield the formation of peptide bonds between Phe-pNA and acyl donors including those with free carboxylic group and non-protected C-terminal basic and acidic amino acid residues.

Charge reversal of ammodytoxin A, a phospholipase A2-toxin, does not abolish its neurotoxicity.

The positive charge concentrated at the C-terminal region of ammodytoxin (Atx) A, which is involved in presynaptic toxicity, has been reversed. A six-site mutant of AtxA (K108N/K111N/K127T/K128E/E129T/K132E , where K108N=Lys(108)-->Asn etc. ) was prepared, in which five out of seven C-terminal basic amino acid residues were substituted with neutral or acidic ones. The mutant was approximately 30-fold less lethal, but still neurotoxic. Consistent with this, its binding affinity for the neuronal receptors decreased by only a factor of five. Additionally, a single-site mutant of AtxA was prepared, with substitution at only one position (K127T) out of six mutated in the six-site mutant. Its toxicity indicated that most, if not all, of the six mutated residues partially contribute to the decreased lethality of the multiple-site mutant.

Charge reversal of ammodytoxin A, a phospholipase A2-toxin, does not abolish its neurotoxicity

The positive charge concentrated at the C-terminal region of ammodytoxin (Atx) A, which is involved in presynaptic toxicity, has been reversed. A six-site mutant of AtxA (K108N/K111N/K127T/K128E/E129T/K132E, where K108N = Lys108 → Asn etc.) was prepared, in which five out of seven C-terminal basic amino acid residues were substituted with neutral or acidic ones. The mutant was approximately 30-fold less lethal, but still neurotoxic. Consistent with this, its binding affinity for the neuronal receptors decreased by only a factor of five. Additionally, a single-site mutant of AtxA was prepared, with substitution at only one position (K127T) out of six mutated in the six-site mutant. Its toxicity indicated that most, if not all, of the six mutated residues partially contribute to the decreased lethality of the multiple-site mutant.

Disruption of the KEX1 gene in Pichia pastoris allows expression of full-length murine and human endostatin.

Endostatin is a potent angiogenesis inhibitor. In order to isolate sufficient quantities of soluble protein for in vivo studies in mice, we expressed murine endostatin in Pichia pastoris. Analysis of the expressed protein by mass spectrometry indicated that the protein was truncated. N-terminal sequence analysis determined that the N-terminus was intact, suggesting that the C-terminal lysine was missing. In Saccharomyces cerevisiae, Kex1p can cleave lysine and arginine residues from the C-terminus of peptides and proteins. We hypothesized that the KEX1 homologue in P. pastoris is responsible for the loss of the C-terminal lysine of endostatin. To test this hypothesis, we cloned and disrupted the P. pastoris KEX1 gene. Although the overall amino acid identity between the P. pastoris and the S. cerevisae Kex1p is only 36%, the amino acid residues involved in the catalytic activity or close to the active residues are highly conserved. Disruption of the KEX1 reading frame allowed expression of murine and human endostatin with the C-terminal lysine. The KEX1 disruption strain may be a useful tool for the expression of other proteins with a C-terminal basic amino acid. Addition of a lysine to the C-terminus of recombinant proteins may protect the C-terminus from degradation by other carboxypeptidases.

Conservation of an intact vif gene of human immunodeficiency virus type 1 during maternal-fetal transmission.

The human immunodeficiency virus type 1 (HIV-1) vif gene is conserved among most lentiviruses, suggesting that vif is important for natural infection. To determine whether an intact vif gene is positively selected during mother-to-infant transmission, we analyzed vif sequences from five infected mother-infant pairs following perinatal transmission. The coding potential of the vif open reading frame directly derived from uncultured peripheral blood mononuclear cell DNA was maintained in most of the 78,912 bp sequenced. We found that 123 of the 137 clones analyzed showed an 89.8% frequency of intact vif open reading frames. There was a low degree of heterogeneity of vif genes within mothers, within infants, and between epidemiologically linked mother-infant pairs. The distances between vif sequences were greater in epidemiologically unlinked individuals than in epidemiologically linked mother-infant pairs. Furthermore, the epidemiologically linked mother-infant pair vif sequences displayed similar patterns that were not seen in vif sequences from epidemiologically unlinked individuals. The functional domains, including the two cysteines at positions 114 and 133, a serine phosphorylation site at position 144, and the C-terminal basic amino acids essential for vif protein function, were highly conserved in most of the sequences. Phylogenetic analyses of 137 mother-infant pair vif sequences and 187 other available vif sequences from HIV-1 databases revealed distinct clusters for vif sequences from each mother-infant pair and for other vif sequences. Taken together, these findings suggest that vif plays an important role in HIV-1 infection and replication in mothers and their perinatally infected infants.

Cellular carboxypeptidases.

This article focuses on four human carboxypeptidases (CPs): two metallo-CPs and two serine CPs. The metallo-CPs are members of the so-called B-type regulatory CP family, as they cleave only the C-terminal basic amino acids Arg or Lys. The plasma membrane-bound CPM and the mainly, but not exclusively, intracellular CPD are surveyed from this group of enzymes. These enzymes can regulate peptide hormone activity at the cell surface and possibly intracellularly after receptor-mediated endocytosis and may also participate in peptide hormone processing. The serine CPs, as their name indicates, contain a serine residue in the active center essential for catalytic activity that reacts with organophosphorus inhibitors. Prolylcarboxypeptidase (PRCP) (angiotensinase C) and deamidase (cathepsin A, lysosomal protective protein) are discussed here. These two enzymes are highly concentrated in lysosomes; however, they may also be active extracellularly after their release from lysosomes in soluble form or in a plasma membrane-bound complex. Whereas deamidase cleaves a variety of peptides with C-terminal or penultimate hydrophobic residues (e.g. substance P, angiotensin I, bradykinin, endothelin, fMet-Leu-Phe). PRCP cleaves only peptides with a penultimate Pro residue (e.g. des-Arg9-bradykinin, angiotensin II). These enzymes may also be involved in terminating signal transduction by inactivating peptide ligands after receptor endocytosis.

The specificity of carboxypeptidase Y may be altered by changing the hydrophobicity of the S'1 binding pocket.

The S'1 binding pocket of carboxypeptidase Y is hydrophobic, spacious, and open to solvent, and the enzyme exhibits a preference for hydrophobic P'1 amino acid residues. Leu272 and Ser297, situated at the rim of the pocket, and Leu267, slightly further away, have been substituted by site-directed mutagenesis. The mutant enzymes have been characterized kinetically with respect to their P'1 substrate preferences using the substrate series FA-Ala-Xaa-OH (Xaa = Leu, Glu, Lys, or Arg) and FA-Phe-Xaa-OH (Xaa = Ala, Val, or Leu). The results reveal that hydrophobic P'1 residues bind in the vicinity of residue 272 while positively charged P'1 residues interact with Ser297. Introduction of Asp or Glu at position 267 greatly reduced the activity toward hydrophobic P'1 residues (Leu) and increased the activity two- to three-fold for the hydrolysis of substrates with Lys or Arg in P'1. Negatively charged substituents at position 272 reduced the activity toward hydrophobic P'1 residues even more, but without increasing the activity toward positively charged P'1 residues. The mutant enzyme L267D + L272D was found to have a preference for substrates with C-terminal basic amino acid residues. The opposite situation, where the positively charged Lys or Arg were introduced at one of the positions 267, 272, or 297, did not increase the rather low activity toward substrates with Glu in the P'1 position but greatly reduced the activity toward substrates with C-terminal Lys or Arg due to electrostatic repulsion. The characterized mutant enzymes exhibit various specificities, which may be useful in C-terminal amino acid sequence determinations.

Cloning and Expression of Human Carboxypeptidase Z, a Novel Metallocarboxypeptidase

A novel cDNA, designated carboxypeptidase Z (CPZ), was identified based on its homology to known metallocarboxypeptidases. Northern blot analysis shows bands of 2.1 and/or 2.6 kilobases in all tissues examined. The major form of CPZ mRNA in human salivary gland encodes a protein with an open reading frame of 641 amino acids. In addition, three variants were found that presumably arise due to alternative intron splicing. The 641-amino acid protein contains an 18-residue signal peptide-like sequence, a 120-residue region that shows 23-29% amino acid identity with a Cys-rich domain found in frizzled proteins and in type XVIII collagen, and then a 390-residue carboxypeptidase domain with 49% amino acid identity to carboxypeptidases E and N. The 641-amino acid form of CPZ expressed in the baculovirus system cleaves 5-dimethylaminonaphthalene-1-sulfonyl (dansyl)-Phe-Ala-Arg, although the level of enzyme activity was approximately 10-fold lower than either carboxypeptidase E or D expressed using the same viral system. The CPZ activity is more active at neutral pH than at pH 5.5 and is inhibited by active site-directed inhibitors of metallocarboxypeptidases. In summary, CPZ is a novel metallocarboxypeptidase that is active toward substrates with C-terminal basic amino acids.

Circular-dichroism and fluorescence studies on melittin: effects of C-terminal modifications on tetramer formation and binding to phospholipid vesicles

Studies were performed on a series of melittin analogues with selective alterations to the positively charged amino acid sequence at the C-terminus. Fluorescence studies were undertaken using the sole tryptophan residue in the analogues as an intrinsic fluorescence probe for indications of tetramer formation in free solution, and binding and insertion of the melittins into phospholipid bilayers. Studies were performed under conditions of low-salt buffer with increasing concentrations of phosphate added to promote self-association of the melittin monomers, and also in the presence of phospholipid vesicles. C.d. studies were also performed under conditions of increasing phosphate concentrations and in the presence of lipid vesicles to monitor the alpha-helical content of the melittins. It was found that selective replacement of the C-terminal basic amino acids by glutamine has different effects on self-association, alpha-helix formation and lipid binding of melittin.

Carboxypeptidase M in brain and peripheral nerves.

Carboxypeptidase M (CPM), a plasma membrane-bound enzyme, cleaves C-terminal basic amino acids with a neutral pH optimum. We studied its distribution in human, baboon, and dog brain and in dog peripheral nerves. Areas were dissected, homogenized, centrifuged, and assayed for activity with dansyl-Ala-Arg. The corpus callosum and the pyramidal and optic tract were especially rich in CPM, whereas basal ganglia and cortex had low activity. The identity of the basic carboxypeptidase activity with CPM was shown by similarities in subcellular localization, membrane attachment, substrate hydrolysis, inhibition by a specific basic carboxypeptidase inhibitor, and cross-reaction with anti-human CPM antiserum. This antiserum immunoprecipitated an average of 85% of the activity in human and baboon brain and approximately 66% in dog brain. CPM co-purified with myelin extracted from the brain. Consistent with results obtained in placenta and cultured kidney cells, CPM in the brain appears to be membrane-bound via a phosphatidylinositol glycan anchor. In the peripheral nerves, the specific activity in dog sciatic nerve and in vagus was high (98 and 149 nmol/h/mg of protein, respectively). In immunohistochemical studies, glia in the brain, which appear to be oligodendrocytes or astrocytes, and the outer aspects of myelin sheaths and Schwann cells in sciatic and vagus nerves were stained. We conclude that in some areas of the CNS and the PNS, CPM is closely associated with myelin and myelin-forming cells. Northern blot analysis revealed the presence of mRNA coding for CPM in the brain, showing that the enzyme is indeed synthesized there.

Carboxypeptidase H processing and secretion in rat clonal beta-cell lines.

Carboxypeptidase H (CP-H, EC 3.4.17.10) removes C-terminal basic amino acids from insulin and many other peptide hormone intermediates in endocrine and neural tissues. CP-H was studied in two rat insulinoma-derived clonal beta-cell lines that were chosen in part because their profile of hormone production is restricted. Although RIN 1046-38 cells predominantly produce insulin and RIN 1046-44 cells contain no known polypeptide hormone, both cell lines contain comparable levels of CP-H. The distribution of CP-H in RIN cells differs from primary tissues since only 25% of cellular CP-H is soluble whereas approximately 75% is associated with the membrane fraction. Immunoblotting with both N- and C-terminal antibodies detects a single 54 kilodalton (kDa) band corresponding to the mature form of CP-H in soluble and membrane RIN cell extracts and incubation media. Pulse-chase analysis shows that a 56 kDa precursor is initially synthesized and rapidly processed to a mature 54 kDa form. Newly synthesized, mature CP-H is detectable in the media by 30 min. Approximately 50% is retained within cells after 3 h of incubation. Secretion of CP-H activity from both RIN cell types and insulin from 1046-38 cells is acutely stimulated by membrane depolarization with KCl and attenuated by 10 mM MgCl2. Dense core secretory vesicles characteristic of the regulated secretory pathway are evident on transmission electron microscopy of both RIN cell lines. Thus, mature CP-H exists primarily as a membrane-associated form in RIN cells, but all forms of CP-H, including soluble and secreted enzyme, retain an intact C-terminus.

Processing and secretion of human carboxypeptidase E by C6 glioma cells

Carboxypeptidase E (CPE) catalyses the removal of C-terminal basic amino acids and is implicated in the processing of peptides and hormones in secretory vesicles. The enzyme has been isolated in multiple forms from both soluble and membrane-bound compartments, and has been demonstrated to be co-secreted with peptides from pancreatic and adrenal cells. To address the question regarding the origin of the multiple forms of the enzyme, we have transfected a construct containing the cDNA for human CPE under the control of the murine-sarcoma-virus enhancer and metallothionein promoter into the C6 rat glioma cell line, which itself has extremely low levels of CPE expression. Within transfectants, multiple forms of the enzyme have been detected by Western blotting, and their sizes are comparable with that in pituitary. Fractionation of the intracellular forms of CPE with Triton X-114 at various pH values indicates that the membrane-bound, but not the soluble, form is amphipathic; this difference probably arises from post-translational modification of the enzyme. Interestingly, the secreted enzyme is found to have the amphipathic characteristics of the membrane-bound form. By modification of the cDNA sequence, enzyme lacking N-terminal and C-terminal domains have been expressed: deletion of the 'pro' region of CPE, leading to loss of the signal cleavage site, gives a more hydrophobic but active enzyme which is not efficiently secreted from the cell; deletion of a C-terminal region gives a secreted form of the enzyme which no longer exhibits amphipathic behaviour. Our data indicate that a single mRNA species can give rise to the multiple forms of CPE enzyme that have been identified and that the C6 cells are able to carry out the intracellular processing events to generate these forms.

Metabolism of Pro-Luteinizing Hormone-Releasing Hormone in Immortalized Hypothalamic Neurons*

An immortalized hypothalamic neuronal cell line was recently developed by genetically targeting the expression of the simian virus-40 large T-antigen in LHRH neurons. These GT1 cells were subcloned to GT1-1, GT1-3, and GT1-7 cells, and they have been shown to express the mRNA for pro-LHRH and secrete LHRH-like immunoreactive (IR) materials into the media. The purpose of our study was to biochemically and immunologically characterize the IR materials within and secreted from these cells. Both LHRH- and GnRH-associated peptide (GAP)-like IR materials were present and were secreted from these four cell lines. Up to 3% of the total cellular protein was composed of LHRH and GAP materials. When materials from the cell lysate and media were separated according to mol wt (Mr), at least three different pro-LHRH species were detected. These precursors contained both LHRH- and GAP-like IR determinants, and they eluted in the void volume and at approximately 10,000-12,000 and 8,400-8,500 Mr. A material that contained GAP-like IR eluted at approximately 6,500-6,800 Mr. This species is probably mouse GAP-(1-56) because it eluted on a reverse phase column in the approximate position of rat GAP-(1-56). Cell lysates contained a single LHRH-like IR form which coeluted on a size-exclusion column with synthetic LHRH. This material stimulated secretion of LH from anterior pituitary cells in a dose-response manner. By comparison, two different molecular forms of LHRH were detected in media at approximately 1,500 and 540 Mr. HPLC analyses revealed these peaks to be heterogeneous and to contain at least (Gln1)LHRH-(Gly11,Lys12,Arg13), (Gln1)LHRH-(Gly1,Lys12), LHRH-(Gly11), and LHRH. These experiments demonstrate that the cells contain and secrete multiple molecular forms of the pro-LHRH and that processing of the prohormone must involve 1) cleavage by an endopeptidase to give GAP-(1-56) and a C-terminally extended LHRH, 2) removal of C-terminal basic amino acids by a carboxypeptidase, 3) amidation of LHRH-(Gly11) to LHRH, and 4) cyclization of glutamine to pyroglutamate at the N-terminal of LHRH. These results provide the first evidence for intermediates in the metabolic pathway of pro-LHRH to LHRH.

Distribution of carboxypeptidase H messenger RNA in rat brain using in situ hybridization histochemistry: implications for neuropeptide biosynthesis

Carboxypeptidase H is an exopeptidase which is highly specific for C-terminal basic amino acids and thought to play a role in neuropeptide biosynthesis. The distribution of carboxypeptidase H mRNA was examined in adult rat brain using in situ hybridization histochemistry. Enzyme transcripts were detected in all major brain areas. Very high levels of carboxypeptidase H mRNA were found in the hippocampus associated with pyramidal cells. Other brain regions showed varied levels of labelling, ranging from high levels in the magnocellular cells of hypothalamic nuclei to very low levels in the striatum. The appearance of enzyme transcripts throughout brain supports a role for this enzyme in the biosynthesis of many neuropeptides. The expression of transcripts in certain ventricular ependymal cells identified them as a new potential peptidergic cell type. The variations in levels of expression of carboxypeptidase H mRNA may reflect differences in peptidergic activity in different neuronal systems.

Carboxypeptidase E.

Carboxypeptidase E appears to be involved in the biosynthesis of a wide range of peptide hormones and neurotransmitters. The evidence for this is: (a) CPE is present in tissues that produce bioactive peptides; (b) in tissues that have been subjected to subcellular fractionation, the CPE activity is associated with peptide-containing secretory granules; (c) CPE is able to remove C-terminal basic amino acids from a variety of synthetic peptides without further hydrolyzing the peptide; (d) CPE is active at pH 5.6, the internal pH of secretory granules. The CPE activities in various tissues have similar physical and enzymatic properties. Two forms of CPE, soluble and membrane-bound, are present in most tissues with CPE activity. These two forms differ slightly in molecular weight, but have identical enzymatic properties. Both forms arise from the same precursor, which is encoded by a single gene. This gene is a member of a carboxypeptidase gene family that includes CPA and CPB. At the amino acid level, CPE has approximately 20% homology with bovine CPA and 17% homology with bovine CPB. All of the amino acids in CPA and CPB that are thought to be essential for catalytic activity are present in CPE in comparable positions. The homology of CPE with CPA and CPB suggests a common evolutionary origin for the three enzymes. This relationship fits with the theory that certain peptide hormones may have evolved from serine proteases. Further studies are needed to investigate the processing of proCPE into CPE, and the regulation of CPE activity. While there is some evidence that CPE may be regulated, it does not appear that regulation of CPE activity plays an important role in controlling peptide biosynthesis. However, further studies are necessary before this possibility can be eliminated.