Peptide decomposition by extracellular hydrolysis in coastal seawater and salt marsh sediment

Extracellular peptide hydrolysis rates were measured in seawater and sedime nt from Flax Pond salt marsh using peptide analogs (LYA-peptides) as substr ates. This technique allows the direct measurement of specific hydrolysis p roducts and thus provides insights into enzymatic hydrolysis pathways. In s ediments, hydrolysis rate constants of LYA-peptides varied seasonally and w ith depth. Highest activity was found in spring and summer, and most cores exhibited a subsurface maximum. Calculations using the concentrations of ch emically-measured peptides suggested that extracellular hydrolysis of pepti des is faster than the rate of free amino acids uptake. However, not all pe ptides may be available for enzymatic hydrolysis. In both seawater and sedi ment, extracellular hydrolysis of peptides of up to 8 amino acids yielded s maller peptides and free amino acids. Hydrolysis rates depended on size of the peptide substrate, although a clear relationship with number of amino a cid constituents was not evident. Peptides containing > 2 amino acids were hydrolyzed 10-400 times faster than dipeptides or the fluorogenic substrate Leucine-MCA. Thus, dipeptidases are either uncommon in nature, or hydrolys is is carried out by nonspecific hydrolases that with a low affinity for di peptides. This is also suggested by the presence of a lag time before dipep tide hydrolysis begins, and the absence of dipeptide hydrolysis in 0.2-mu m -filtered. One implication of this finding is that measurements of hydrolys is rates using substrates like Leu-MCA may not accurately predict the magni tude of hydrolysis rates of macromolecules in the marine environment. Even though dipeptide hydrolysis is slow compared to that of larger peptides, LY A-dipeptides are preferentially produced from the hydrolysis of larger subs trates. LYA-dipeptides do not penetrate cell membranes of microorganism bec ause of their size, but natural dipeptides are smaller and can be transport ed across the cell membrane. Since dipeptides do not appear to accumulate i n natural waters, they must be rapidly removed by microorganisms. (C) 1999 Elsevier Science B.V. All rights reserved.