An aspect of special interest is the in vivo dynamics of each of the dipeptides, since the mode or site of hydrolysis determines the capacity and efficiency of their utilization.
Recent studies using novel in vitro assays to measure hydrolase activity against glutamine-, tyrosine-, or cystine-containing dipeptides in cellular fractions from plasma, mucosa, and muscle demonstrated that dipeptides They are substrates for free peptidases (plasma and cytos1) and also for peptidases bound to the cell membrane.(84-87)
The rate of plasma hydrolysis depends on the composition of amino acids of the peptides and it is necessary to consider both the N-terminal and C-terminal amino acids. The capacity of the plasma hydrolase for alanyl-glutamine is about 20 to 40 nmol/mL min.
This high capacity could partly explain the extremely short plasma half-life observed in vivo in previous studies.(49,51) The affinity towards alanyl-glutamine of membrane-bound peptidases, in the basolateral membrane, the of the brush border and the muscular sarcolemma is very similar to that observed in the plasma and cytosol.
The elimination of intravenously administered alanyl-glutamine is the combined result of hydrolysis catalyzed by free peptidases in plasma and membrane-bound peptidases.The latter demonstrated a greater affinity and capacity against alanyl-glutamine than against glycyl-glutamine, which explains the prolonged elimination period in vivo.
All these observations indicate that the structure of the peptides influences the elimination half-life and also the site of hydrolysis.
It is necessary to define the nature and specification of the most favorable peptides or groups of peptides.
Although rapid elimination of intravenously administered peptides could reduce the risk of undesirable pharmacological or physiological side effects,(88) it is still too early to make explicit recommendations.
It is very likely that the use that a given tissue can make of the peptide varies according to the structure and biological effects of the latter.
We do not believe that it is useful to speculate on the correlation between the structure of the peptide and its affinity for hydrolysis.( 7.89)
To properly evaluate the influence of the structure on the behavior in hydrolysis it would be necessary to consider the effects of about 400 possible structural variations.
Considering the intention to improve clinical nutrition through the use of intravenous dipeptides, the marginal differences with respect to half-lives are not of great importance.
Much more important is to demonstrate the selective or specific use of a given dipeptide or amino acid/dipeptide combination by different organs.
The future of peptides in parenteral nutrition
Recently, the possibility of using short chain peptides as additional or alternate substrates for amino acids within the context of parenteral nutrition.
The low osmolarity of parenteral peptide-based solutions allows nitrogen needs to be met with a low volume in cases where fluid restriction is desirable. (7)
Adding dipeptides containing glutamine, tyrosine and cystine is an important step towards solving the problem of how to formulate and prepare new amino acid solutions adapted to the patient’s needs. (90)
The objective of this article was to analyze the nutritional potential of short chain peptides and to emphasize the need for glutamine peptides, especially in certain catabolic conditions.
It is too early to make a recommendation regarding the nature and specification of the most favorable peptide or groups of peptides.
It is very likely that the The ability of tissues to use a peptide specifically directed at them varies according to the structure and biological effects of the peptide in question.(45,52,84,91,92) There are certain diseases that can generate deficiencies,(15,47 ) antagonisms or imbalances of certain amino acids in the various organic tissues.
In these conditions it may be necessary to selectively administer one or more specific peptides required only in each particular situation to support the tissues where there is a shortage of them.
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