The identification of an eukaryotic initiation factor 4D precursor in spermidine-depleted Chinese hamster ovary cells

When Chinese hamster ovary cells are incubated with [terminal methylenes-3H]spermidine, radioactivity is incorporated into a single cellular protein, eukaryotic initiation factor 4D (eIF-4D), through posttranslational synthesis of the amino acid hypusine (N epsilon-(4-amino-2-hydroxybuyly)lysine). The effect of spermidine depletion on this protein modification reaction was studied by high resolution two-dimensional gel electrophoresis. Factor eIF-4D containing both [3H]lysine and [3H]hypusine was detected as one of the major labeled cellular proteins on the fluorographic map of the proteins from Chinese hamster ovary cells that had been incubated with [3H]lysine. When these cells were depleted of spermidine by the use of DL-alpha-difluoromethylornithine before addition of [3H]lysine, no radiolabeling of this mature eIF-4D (hypusine form, Mr approximately 18,000; pI approximately 5.3) occurred. Instead, a new radiolabeled protein (Mr 18,000; pI 5.1) that contained [3H]lysine but no [3H]hypusine or [3H]deoxyhypusine was seen. This protein was identified as an eIF-4D precursor by comparison of the two-dimensional map of its tryptic peptides with that of the tryptic peptides from [3H]lysine-labeled eIF-4D. Further comparisons also suggest that additional post-translational modification processes are involved in the biogenesis of eIF-4D.     

Comparison of the activities of variant forms of eIF-4D. The requirement for hypusine or deoxyhypusine.

Eukaryotic protein synthesis initiation factor 4D (eIF-4D) (current nomenclature, eIF-5A) contains the unique amino acid hypusine (N epsilon-(4-amino-2-hydroxybutyl)lysine). The first step in hypusine biosynthesis, i.e. the formation of the intermediate, deoxyhypusine (N epsilon-(4-aminobutyl)lysine), was carried out in vitro using spermidine, deoxyhypusine synthase, and ec-eIF-4D(Lys), an eIF-4D precursor prepared by over-expression of human eIF-4D cDNA in Escherichia coli. In a parallel reaction, using N-(3-aminopropyl)cadaverine in place of spermidine, a variant form of eIF-4D containing homodeoxyhypusine (N epsilon-(5-aminopentyl)lysine) was prepared. Evidence that N-(3-aminopropyl)cadaverine can also act as the amine substrate for deoxyhypusine synthase in intact cells was obtained by incubating putrescine- and spermidine-depleted Chinese hamster ovary cells with [3H]cadaverine. In these cells, in which [3H]cadaverine is readily converted to N-(3-aminopropyl) [3H]cadaverine, small amounts of [3H]homodeoxyhypusine and another 3H-labeled compound, presumed to be N epsilon-(5-amino-2-hydroxy[3H]pentyl)lysine, were found. eIF-4D stimulates methionyl-puromycin synthesis, an in vitro model assay for translation initiation. Whereas the unmodified precursor ec-eIF-4D(Lys) appeared inactive, the deoxyhypusine-containing form provided a significant degree of stimulation. The variant form containing homodeoxyhypusine, on the other hand, showed little or no activity. These findings emphasize the importance of hypusine or deoxyhypusine for the biological activity of eIF-4D and demonstrate the influence of both the length and chemical nature of its amino alkyl side chain.     

The mammalian hypusine-containing protein, eukaryotic initiation factor 4D. Structural homology of this protein from several species

A single cellular protein of Mr approximately 18,000 and pI near 5.1, recently identified as eukaryotic translation initiation factor eIF-4D, contains the unusual amino acid hypusine [N epsilon-(4-amino--2-hydroxybutyl)lysine] formed post-translationally from lysine with a structural contribution from the polyamine spermidine. When the 3H-labeled hypusine-containing protein isolated from Chinese hamster ovary (CHO) cells that were grown with radioactive polyamine is digested with trypsin and the digest is subjected to two-dimensional separation, a single radioactive peptide is seen. A labeled peptide that occupies this same position is found in a digest of the [3H]hypusine protein from human lymphocytes and the single hypusine-containing tryptic peptide from purified rabbit reticulocyte eIF-4D also moves to this identical position. Stepwise Edman degradation of the tryptic digest of CHO cell hypusine-protein releases the radioactivity as a single peak in accordance with our earlier evidence for a single hypusine residue per molecule of eIF-4D. The similar patterns of radioactive peptides obtained from tryptic digests of radioiodinated eIF-4D from CHO cells, human lymphocytes, and rabbit reticulocytes suggest a highly conserved primary structure for this protein.     

The biosynthesis of hypusine (N epsilon-(4-amino-2-hydroxybutyl)lysine). Alignment of the butylamine segment and source of the secondary amino nitrogen

The unusual amino acid hypusine is produced in a single protein of mammalian cells by a novel posttranslational event in which a lysine residue is conjugated with the four-carbon moiety from the polyamine spermidine to form an intermediate deoxyhypusine, and in which this intermediate is subsequently hydroxylated. Specifically isotopically labeled precursors of hypusine were used to identify the biosynthetic origin of some of the atoms of hypusine and thus to provide further insight into the mechanism of this in vivo chemical modification reaction. Radiolabel from [1,4-3H] putrescine, [1,8-3H]spermidine, and [5-3H]spermidine entered hypusine during growth of Chinese hamster ovary cells. The occurrence of this label at positions 1 and 4, at position 4, and at position 1, respectively, in the 4-amino-2-hydroxybutyl portion of hypusine revealed an alignment of atoms identical to that in the butylamine segment of spermidine. Growth of cells with [epsilon-15N]lysine as the source of lysine yielded hypusine enriched in 15N, whereas only isotope-free hypusine during growth by [4-15N]spermidine. These was found in cells whose spermidine was replaced during growth by [4-15N]spermidine. These findings are in accordance with a proposal that the first phase of hypusine biosynthesis, the production of intermediate deoxyhypusine, occurs through transfer of the butylamine moiety from spermidine to the epsilon-amino nitrogen of protein-bound lysine. The technique of thermospray high-performance liquid chromatography/mass spectrometry provided positive identification of 15N in hypusine through final separation and on-column direct analysis of this amino acid. Methods of preparation are given for spermidine of high specific radioactivity, labeled specifically at position 5 with 3H, and for spermidine with 15N at the 4-position.     

Identification of Posttranslationally Modified 18-Kilodalton Protein from Rice as Eukaryotic Translation Initiation Factor 5A

Using anther-derived rice (Oryza sativa L.) cell-suspension cultures, we have identified an 18-kD protein that is posttranslationally modified by spermidine and is influenced by endogenous polyamine levels. The posttranslationally modified residue has been identified as the unusual amino acid hypusine [N[epsilon]-(4-amino-2-hydroxybutyl)lysine] by reverse-phase high-performance liquid chromatography and gas chromatography-mass-spectrometry analyses. Differential labeling of the protein with labeled amines provided evidence that the butylamine moiety of spermidine is the immediate precursor of the hypusine residue in the protein. The eukaryotic translation initiation factor 5A (eIF-5A) is the only known mammalian protein that undergoes a similar posttranslational modification with hypusine. The purified 18-kD protein co-electrophoreses with human translational initiation factor eIF-5A in both isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gels. The purified protein from rice stimulated methionyl-puromycin synthesis in vitro, indicating its functional similarity to mammalian eIF-5A. The results presented provide evidence that the posttranslationally modified 18-kD protein from rice containing hypusine is eIF-5A and suggest the conservation of hypusine-containing translation initiation factor eIF-5A in eukaryotes.     

The archaebacterial hypusine-containing protein. Structural features suggest common ancestry with eukaryotic translation initiation factor 5A.

The amino acid hypusine is formed by post-translational modification of a lysine residue in eukaryotes and archaebacteria but up to now only the eukaryotic translation initiation factor eIF-5A has been known to contain this unique component. We isolated and purified a hypusine-containing protein from the thermophilic archaebacterium Sulfolobus acidocaldarius. The mainly cytosolic protein comprised about 0.03% of the post-ribosomal supernatant protein. No other hypusine-containing protein could be detected in S. acidocaldarius. The molar ratio of hypusine/hypusine-containing protein was 1:1. SDS/PAGE showed a molecular mass of 16.8 kDa; a pI of 7.8 for the native protein resulted from IEF. The N-terminus was blocked. Four cyanogen bromide fragments were partially sequenced and used to derive two 17-base oligonucleotide probes. A 3-kb HindIII fragment of genomic DNA hybridizing with both probes was cloned. By sequencing of exonuclease III deletion clones an open reading frame of 405 nucleotides was found coding for a protein of 135 amino acids with a molecular mass of 15 kDa. It contained all cyanogen bromide sequences analysed. Sequence alignment revealed that seven of eight residues around Lys40 in the Sulfolobus hypusine-containing protein were identical to the nonapeptides centered by hypusine in the three eIF-5A proteins sequenced so far. The Edman procedure gave no phenylthiohydantoin derivative for this position. For a central region of 44 residues a sequence similarity of 54% between the archaebacterial and eukaryotic proteins was calculated; for the total sequence about 33% similarity resulted. In addition, there were a number of conservative changes. The unique lysine modification surrounded by a conserved sequence strongly suggests a common ancestry of archaebacterial hypusine-containing protein and eIF-5A. Together with similarities in molecular mass and intracellular localization, it may point to an analogous biochemical function.     

Eukaryotic initiation factor 4D, the hypusine-containing protein, is conserved among eukaryotes

When mammalian cells are grown in medium containing [3H]spermidine, a single major tritiated protein identical to eukaryotic initiation factor 4D becomes labeled. This protein contains 1 residue/molecule of tritiated hypusine (N epsilon-(4-amino-2-hydroxybutyl)lysine), a rare amino acid which has been found in no other protein. In order to investigate the conservation of this protein, we examined two nonmammalian eukaryotes, the yeast Saccharomyces cerevisiae and the insect Drosophila melanogaster, and the eubacterial prokaryote Escherichia coli for the presence of the hypusine-containing protein. When the eukaryotic cells were grown in the presence of [3H]spermidine, electrophoretic analysis revealed a single labeled protein. In each case, the apparent molecular weight was near 18,000 and the relative pI was approximately 5.2, similar to the hypusine-containing protein of mammals. Amino acid analysis confirmed the presence of tritiated hypusine in each case, and silver staining of two-dimensional polyacrylamide gels demonstrated that, in yeast and fruit flies as in mammals, the protein is relatively abundant. In the eubacterium E. coli, one tritiated protein was predominant, but its molecular weight was 24,000 and we found no evidence that it contained tritiated hypusine. We found no evidence for the existence of the hypusine-containing protein in the archaebacterium Methanococcus voltae. These data suggest that the hypusine-containing protein is conserved among eukaryotes.     

The essential role of hypusine in eukaryotic translation initiation factor 4D (eIF-4D). Purification of eIF-4D and its precursors and comparison of their activities

Eukaryotic translation initiation factor 4D (eIF-4D) is the only protein known to contain the amino acid, hypusine [N epsilon-(4-amino-2-hydroxybutyl)lysine]. This unusual amino acid is formed post-translationally by modification of a single specific lysine residue in an eIF-4D precursor protein. Two separate eIF-4D precursors, each of which contains a lysine residue in place of the hypusine residue and each of which thereby serves as a protein substrate for the hypusine modification, were purified from DL-2-difluoromethylornithine-treated Chinese hamster ovary cells by means of a five-step procedure. These two precursors termed PI and PII both have apparent molecular masses of approximately 17 kDa, indistinguishable from that of eIF-4D, but exhibit more acidic isoelectric points (5.1 and 5.25 for PI and PII, respectively, compared with 5.37 for eIF-4D). These physical characteristics, together with other properties, indicate that eIF-4D differs from PII only in possessing the hypusine residue in place of a lysine residue, whereas an additional structural difference exists between PI and eIF-4D. eIF-4D from CHO cells provides a significant enhancement of methionyl-puromycin synthesis, a model assay for translation initiation. Neither PI nor PII stimulates this in vitro system. These findings are the first direct evidence that hypusine is essential for the biological activity of eIF-4D.     

Characterization and reconstitution of a cell free system for NAD(+)-dependent deoxyhypusine formation on the 18 kDa eIF-4D precursor

Deoxyhypusine formation on the 18 kDa eIF-4D precursor is due to a covalent linkage between a lysine residue of the protein and the aminobutyl moiety derived from spermidine. The deoxyhypusine is then hydroxylated to form hypusine. This post-translational modification represents one of the most specific spermidine-dependent biochemical events in eukaryotic cells. Deoxyhypusine formation can be performed in vitro at pH 9.5 and is greatly stimulated by NAD+. Using the labeling of the 18 kDa protein by [3H]spermidine as an assay for deoxyhypusine formation, we found that (i) significant deoxyhypusine formation can be demonstrated in vitro at pH 7.2 only if NAD+ is present, (ii) deoxyhypusine formation was sensitive to buffer composition; buffers made of basic amino acids and Tris were inhibitory, (iii) sulfhydryl reagents and metal ions such as Cu2+ and Fe3+ were potent inhibitors of deoxyhypusine formation and (iv) the 18 kDa protein substrate was heat-stable. The in vitro activity of deoxyhypusine formation, which depends on the presence of both enzyme and protein substrate, can be separated from the product, eIF-4D, by a one-step Cibacron blue dye affinity column. Taking advantage of this finding, we have developed a simple procedure, based on the use of Cibacron blue dye, for partially purifying both the deoxyhypusine-forming enzyme and the 18 kDa protein substrate. When the partially purified enzyme and protein substrate were mixed in the presence of 1 mM NAD+ and [3H]spermidine, the 18 kDa protein was radiolabeled, no labeling could be detected if any one component was absent. Using partially purified enzyme, we have also determined the half-life of the protein substrate in alpha-difluoromethyl ornithine (DFMO)-treated NB-15 cells and found it to be longer than 10 h.     

The polyamine-derived amino acid hypusine: its post-translational formation in eIF-5A and its role in cell proliferation

Summary The unusual amino acid hypusine [N -(4-amino-2-hydroxybutyl)lysine] is a unique component of one cellular protein, eukaryotic translation initiation factor 5A (eIF-5A, old terminology, eIF-4D). It is formed posttranslationally and exclusively in this protein in two consecutive enzymatic reactions, (i) modification of a single lysine residue of the eIF-5A precursor protein by the transfer of the 4-aminobutyl moiety of the polyamine spermidine to its-amino group to form the intermediate, deoxyhypusine [N -(4-aminobutyl)lysine] and (ii) subsequent hydroxylation of this intermediate to form hypusine. The amino acid sequences surrounding the hypusine residue are strictly conserved in all eukaryotic species examined, suggesting the fundamental importance of this amino acid throughout evolution. Hypusine is required for the activity of eIF-5Ain vitro. There is strong evidence that hypusine and eIF-5A are vital for eukaryotic cell proliferation. Inactivation of both of the eIF-5A genes is lethal in yeast and the hypusine modification appears to be a requirement for yeast survival (Schnier et al., 1991 [Mol Cell Biol 11: 3105–3114]; Wöhl et al., 1993 [Mol Gen Genet 241: 305–311]). Furthermore, inhibitors of either of the hypusine biosynthetic enzymes, deoxyhypusine synthase or deoxyhypusine hydroxylase, exert strong anti-proliferative effects in mammalian cells, including many human cancer cell lines. These inhibitors hold potential as a new class of anticancer agents, targeting one specific eukaryotic cellular reaction, hypusine biosynthesis.     

Isolation and characterization of an 18 kDa hypusine-containing protein from cultured NB-15 mouse neuroblastoma cells

An 18 kDa protein can be metabolically labeled by [3H]putrescine or [3H]spermidine in various mammalian cells. The labeling is due to a post-translational modification of one lysine residue to hypusine using the aminobutyl moiety derived from spermidine. In view of the lack of knowledge of the function of this spermidine-modified protein, we decided to use the radioactivity associated with the [3H]spermidine-labeled 18 kDa protein as a tracer to develop a simple procedure for purifying this protein from cultured cells. We first screened more than 15 different affinity adsorbents for their ability to bind the labeled 18 kDa protein. This approach enabled us to develop a four-step procedure to purify the labeled 18 kDa protein from NB-15 mouse neuroblastoma cells. The procedure, including a Cibacron Blue column, an omega-aminooctyl-agarose, a Sepharose G-50, and a Mono Q column, resulted in an 800-fold purification of the labeled 18 kDa protein. Two-dimensional gel analysis of fractions enriched in the labeled 18 kDa protein revealed (i) the presence of isoforms of hypusine-containing 18 kDa protein, with pI values ranging from 4.7 to 5.2, and (ii) the presence of an additional labeled protein with an apparent molecular mass of 22 kDa and a pI value of 5.0. The labeling intensity of the 22 kDa protein, however, was less than 5% of that of the 18 kDa protein. Peptide map analysis, using the V-8 proteinase digestion method, indicated that the 18 kDa hypusine-containing protein obtained from NB-15 cells was similar to eukaryotic initiation factor 4D isolated from rabbit reticulocytes.     

Hypusine formation in protein by a two-step process in cell lysates

The putative protein synthesis initiation factor eukaryotic initiation factor 4D (eIF-4D) is post-translationally modified by the polyamine spermidine, forming the rare amino acid hypusine from a lysine residue. The hypusine precursor, deoxyhypusine, was formed in crude cell lysates at pH 9.5 and converted to hypusine at pH 7.1. The modification occurred in eIF-4D, since the isoelectric points and molecular weights of the proteins modified in intact cells and lysates were indistinguishable. Only lysates from cells treated with alpha-difluoromethylornithine, to deplete endogenous polyamine pools, supported the formation of deoxyhypusine, suggesting that unmodified eIF-4D accumulated in spermidine deficient cells. Guazatine, an inhibitor of enzymes which form delta 1-pyrroline from spermidine, blocked deoxyhypusine formation in lysates by nearly 70% at 100 microM and completely at 1 mM. Other mammalian amine oxidase inhibitors had little or no effect on this reaction. Thus, deoxyhypusine formation in eIF-4D is catalyzed by a guazatine-sensitive enzyme with a basic pH optimum.     

Reversal of the deoxyhypusine synthesis reaction. Generation of spermidine or homospermidine from deoxyhypusine by deoxyhypusine synthase

Deoxyhypusine synthase catalyzes the first step in hypusine (N epsilon-(4-amino-2-hydroxybutyl)lysine) synthesis in a single cellular protein, eIF5A precursor. The synthesis of deoxyhypusine catalyzed by this enzyme involves transfer of the 4-aminobutyl moiety of spermidine to a specific lysine residue in the eIF5A precursor protein to form a deoxyhypusine-containing eIF5A intermediate, eIF5A(Dhp). We recently discovered the efficient reversal of deoxyhypusine synthesis. When eIF5A([3H]Dhp), radiolabeled in the 4-aminobutyl portion of its deoxyhypusine residue, was incubated with human deoxyhypusine synthase, NAD, and 1,3-diaminopropane, [3H]spermidine was formed by a rapid transfer of the radiolabeled 4-aminobutyl side chain of the [3H]deoxyhypusine residue to 1,3-diaminopropane. No reversal was observed with [3H]hypusine protein, suggesting that hydroxylation at the 4-aminobutyl side chain of the deoxyhypusine residue prevents deoxyhypusine synthase-mediated reversal of the modification. Purified human deoxyhypusine synthase also exhibited homospermidine synthesis activity when incubated with spermidine, NAD, and putrescine. Thus it was found that [14C]putrescine can replace eIF5A precursor protein as an acceptor of the 4-aminobutyl moiety of spermidine to form radiolabeled homospermidine. The Km value for putrescine (1.12 mM) as a 4-aminobutyl acceptor, however, is much higher than that for eIF5A precursor (1.5 microM). Using [14C]putrescine as an acceptor, various spermidine analogs were evaluated as donor substrates for human deoxyhypusine synthase. Comparison of spermidine analogs as inhibitors of deoxyhypusine synthesis, as donor substrates for synthesis of deoxyhypusine (or its analog), and for synthesis of homospermidine (or its analog) provides new insights into the intricate specificity of this enzyme and versatility of the deoxyhypusine synthase reaction.     

The role of mammalian initiation factor eIF-4D and its hypusine modification in translation

Initiation factor eIF-4D functions late in the initiation pathway, apparently during formation of the first peptide bond. The factor is post-translationally modified at a specific lysine residue by reaction with spermidine and subsequent hydroxylation to form hypusine. A precursor form lacking hypusine is inactive in the assay for methionyl-puromycin synthesis, but activity is restored following in vitro modification to deoxyhypusine, thereby suggesting that the modification is essential for function. Since formylated methionyl-tRNA is less dependent on eIF-4D in the puromycin assay, we postulate that eIF-4D and its hypusine modification may stabilize charged Met-tRNA binding to the peptidyl transferase center of the 60S ribosomal subunit. Analysis of eIF-4D genes in yeast indicate that eIF-4D and its hypusine modification are essential for cell growth.     

Functional significance of eIF5A and its hypusine modification in eukaryotes

The unusual basic amino acid, hypusine [N^ε-(4-amino-2-hydroxybutyl)-lysine], is a modified lysine with the addition of the 4-aminobutyl moiety from the polyamine spermidine. This naturally occurring amino acid is a product of a unique posttranslational modification that occurs in only one cellular protein, eukaryotic translation initiation factor 5A (eIF5A, eIF-5A). Hypusine is synthesized exclusively in this protein by two sequential enzymatic steps involving deoxyhypusine synthase (DHS) and deoxyhypusine hydroxylase (DOHH). The deoxyhypusine/hypusine synthetic pathway has evolved in archaea and eukaryotes, and eIF5A, DHS and DOHH are highly conserved suggesting a vital cellular function of eIF5A. Gene disruption and mutation studies in yeast and higher eukaryotes have provided valuable information on the essential nature of eIF5A and the deoxyhypusine/hypusine modification in cell growth and in protein synthesis. In view of the extraordinary specificity and functional significance of hypusine-containing eIF5A in mammalian cell proliferation, eIF5A and the hypusine biosynthetic enzymes are novel potential targets for intervention in aberrant cell proliferation.     

Biosynthetic labeling of hypusine in mammalian cells. Carbon-hydrogen bond fissions revealed by dual labeling

Using a dual-label technique in which 3H- and 14C-labeled forms of putrescine and of spermidine were employed as biosynthetic precursors of hypusine, two -C-H bond cleavages were detected during production of this unique amino acid in Chinese hamster ovary cells. One of these cleavages occurs at C-1 of the 4-aminobutyl group during its transfer from the secondary amine nitrogen of spermidine to the nitrogen at the epsilon-position of a specific lysine residue in the polypeptide precursor of eukaryotic initiation factor 4D. Breakage of the other -C-H bond takes place at C-2 in this aminobutyl segment after it has been coupled to lysine to form the intermediate deoxyhypusine residue. Hydroxylation at this carbon atom, which constitutes the last step in hypusine biosynthesis, is the cause of bond cleavage. The data obtained are consistent with a notion that no additional -C-H bond fissions occur during hypusine biosynthesis. Our findings permit suggestion of a mechanism for enzymic aminobutyl group transfer in which 4-aminobutyraldehyde produced by oxidative cleavage of spermidine is coupled with the epsilon-amino group of a specific lysine residue to form an enzyme-bound imine intermediate.     

Protozoan hemoglobin from Tetrahymena pyriformis. Isolation, characterization, and amino acid sequence

A hemoprotein that can be defined as hemoglobin based on oxygen binding was isolated from Tetrahymena pyriformis. The protein exists in monomeric form and is separated into four fractions (Ia, Ib, IIa, and IIb) on a CM-cellulose column. From examinations of the absorption spectra and the N-terminal sequence, fractions Ia and Ib were assigned to the oxy-form and its met-form, respectively, of the one protein, while IIa and IIb corresponded to those of the other one. The complete amino acid sequence was therefore determined of fractions I and II. The I was composed of 121 amino acid residues, with the N-terminal serine being blocked. The II, on the other hand, consisted of 119 amino acid residues, its sequence being exactly identical to that of the third residue, lysine, to the C-terminal lysine of the fraction I. Although the genomic multiplicity cannot be ruled out completely, we have concluded that fraction II is a degradation product of the fraction I by endogeneous proteases. The amino acid sequence of T. pyriformis hemoglobin is very unique and showed no notable degree of similarity with the other hemoglobins sequenced so far, but it was found to be 33.9% identical with Paramecium caudatum hemoglobin by a maximal alignment.     

Expression of variant forms of proopiomelanocortin, the common precursor to corticotropin and beta-lipotropin in the rat pars intermedia

Proopiomelanocortin, the common glycoprotein precursor to adrenocorticotropin (ACTH) and beta-lipotropin (beta-LPH), is the most abundant protein synthesized in rat neurointermediate lobes. It represents 30% of the total amount of radioactive proteins obtained after a 1-h pulse incubation with [3H]phenylalanine. Several forms of this protein can be separated by a high-resolution two-dimensional gel electrophoresis technique. The three most abundant species which can be reproducibly characterized by their apparent molecular weights (Mr) and isoelectric points (pI) were called form I (Mr 34 000; pI 8.2), form II (Mr 36 000; pI 8.2), and form III (Mr 35 000; pI 7.3). Additional minor forms, representing together approximately 30% of the total forms I, II, and III combined, are also observed. They have very close molecular weights but differ by their isoelectric points. When glycosylation is prevented by tunicamycin, forms I and II are replaced by a new molecule with the same pI of 8.2 but a slightly lower Mr (32 000). This form is referred to as form T1. Similarly, form III is replaced by form T2 (Mr 33 000; pI 7.3). Forms T1 and T2 are supposed to be nonglycoslyated peptides. They were further characterized by microsequencing and peptide mapping. They both have the same N-terminal amino acid sequence with leucine residues in positions 3 and 11, and they both contain identical [3H]phenylalanine-labeled tryptic fragments, two of them corresponding to the sequences 1-8 of ACTH and 61-69 of beta-LPH. However, a limited digestion with the Staphylococcus aureus (V8 strain) protease generates a collection of peptides different for each form. These results suggest the presence of at least two different gene products corresponding to the major forms of proopiomelanocortin in the rat pars intermedia.     

Cleavage of spermidine as the first step in deoxyhypusine synthesis. The role of NAD

The biosynthesis of deoxyhypusine (N-(4-aminobutyl)lysine) occurs by the transfer of the 4-aminobutyl moiety of spermidine to a specific lysine residue in a precursor of eukaryotic translation initiation factor 4D (eIF-4D). Deoxyhypusine synthase, the enzyme that catalyzes this reaction, was purified approximately 700-fold from rat testis. The Km values for the substrates, spermidine, the eIF-4-D precursor protein, and NAD+, were estimated as approximately 1, 0.08, and 30 microM, respectively. After incubation of partially purified enzyme with [1,8-3H]spermidine, NAD+, and the eIF-4D precursor, equal amounts of radioactivity were found in free 1,3-diaminopropane and in protein-bound deoxyhypusine. However, when the protein substrate (eIF-4D precursor) was omitted, radioactivity was found in 1,3-diaminopropane and in delta 1-pyrroline in nearly equal quantities, providing evidence that the cleavage of spermidine occurs, albeit at a slower rate, in the absence of the eIF-4D precursor. That NAD+, which is required for this reaction, functions as the hydrogen acceptor was demonstrated by the fact that radioactivity from spermidine labeled with 3H at position 5 is found in NADH as well as in delta 1-pyrroline. Transfer of this hydrogen from spermidine to the re face of the nicotinamide ring of NAD+, as determined by the use of dehydrogenases of known stereospecificity, defines the first step of deoxyhypusine synthesis as a pro-R, or A, stereospecific dehydrogenation. Based on these findings, an enzyme mechanism involving imine intermediate formation is proposed.