kinetics-based approach

We developed a biomimetic approach, based on direct incubation with proteolytic enzymes, to measure bioavailable amino acids in sediments. The kinetics of release of monomers and oligopeptides, which are amenable to absorption by cells, is measured as either individual or total amino acids. Microbial proteases incubated with fresh sediments yield amino acids at a similar rate as gut juices from a deposit-feeding holothuroid. Amino acid release from fresh sediment was dominated by a slow hydrolysis step from a refractory substrate, which can be described with a first-order rate law. Typical rate constants for release were in the range 0.15-0.52 h-l, consistent with gut residence times of deposit feeders. The bioavailable pool of peptides had a composition resembling that of average biological tissue, but comparisons with tissue composition suggested dietary deficiency of some essential amino acids. The early stages of release exhibited significant contributions from osmolytes (e.g. glutamic acid and taurine), while a larger, more slowly released pool was anomalously low in methionine, suggesting a contribution from microbial coat proteins. Freezing the sediment led to greater yields of amino acids, which reduced the simulation of a deposit feeder’s gut but indicated a larger pool of potentially bioavailable substrate protected by some kind of matrix. This larger pool of peptide substrate was usually dominated by higher molecular weight polypeptides as measured b