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Background: Urine is supersaturated in calcium oxalate, which means that it will contain calcium oxalate crystals that form spontaneously. Their size must be controlled to prevent retention in ducts and the eventual development of a lithiasis. This is achieved, in part, by specific inhibitors of crystal growth. We investigated whether promoters of crystal nucleation could also participate in that control, because for the same amount of salt that will precipitate from a supersaturated solution, increasing the number of crystals will decrease their average size and facilitate their elimination., Methods: Albumin was purified from commercial sources and from the urine of healthy subjects or idiopathic calcium stone formers. Its aggregation properties were characterized by biophysical and biochemical techniques. Albumin was then either attached to several supports or left free in solution and incubated in a metastable solution of calcium oxalate. Kinetics of calcium oxalate crystallization were determined by turbidimetry. The nature and efficiency of nucleation were measured by examining the type and number of neoformed crystals., Results: Albumin, one of the most abundant proteins in urine, was a powerful nucleator of calcium oxalate crystals in vitro, with the polymers being more active than monomers. In addition, nucleation by albumin apparently led exclusively to the formation of calcium oxalate dihydrate crystals, whereas calcium oxalate monohydrate crystals were formed in the absence of albumin. An analysis of calcium oxalate crystals in urine showed that the dihydrate form was present in healthy subjects and stone formers, whereas the monohydrate, which is thermodynamically more stable and constitutes the core of most calcium oxalate stones, was present in stone formers only. Finally, urinary albumin purified from healthy subjects contained significantly more polymers and was a stronger promoter of calcium oxalate nucleation than albumin from idiopathic calcium stone formers., Conclusions: Promotion by albumin of calcium oxalate crystallization with specific formation of the dihydrate form might be protective, because with rapid nucleation of small crystals, the saturation levels fall; thus, larger crystal formation and aggregation with subsequent stone formation may be prevented. We believe that albumin may be an important factor of urine stability.

The structure of a monoclinic form of bovine pancreatic trypsin inhibitor (BPTI) crystallized from a thiocyanate solution has been determined and refined at 2.7 A resolution. The space group is P21 with a = 71.56, b = 73.83, c = 64.47 A, beta = 93.9 degrees and Z = 20. The ten independent molecules were located by a multi-body molecular-replacement search as developed in the AMoRe program, starting from a single monomer model (PDB code: 6PTI). The molecular arrangement of the subunits is a decamer resulting from the combination of two orthogonal fivefold and twofold non-crystallographic axes. This builds a globular micelle-like particle which minimizes hydrophobic interactions with the solvent. The refinement was conducted with non-crystallographic symmetry constraints up to a final residual of R = 0.20 (Rfree= 0.26). The root-mean-square deviations from ideal geometry were 0.015 A and 1.6 degrees on bond distances and bond angles, respectively. Several sites for thiocyanate ions were analyzed.

Nucleation and crystal growth of hen egg-white lysozyme, bovine pancreatic trypsin inhibitor and porcine pancreatic alpha-amylase were carried out in the presence of a magnetic field of 1.25 T produced by small permanent magnets. Crystals were oriented in the magnetic field, except when heterogeneous nucleation occurred. The orientation of protein crystals in the presence of a magnetic field can be attributed to the anisotropic diamagnetic susceptibility of proteins resulting from the large anisotropy of the alpha-helices due to the axial alignment of the peptide bonds.

Osmotic pressure, small-angle X-ray scattering and quasi-elastic light scattering were used to study the medium-range interaction potentials between macromolecules in solution. These potentials determine macromolecular crystallization. Calf eye lens gamma-crystallins were used as a model system with the charge, and therefore the interactions, varied with pH. The second virial coefficient was determined under the same conditions with each of the three techniques. Osmotic pressure and quasi-elastic light scattering can be used conveniently in the laboratory to rapidly test the type of interactions (either attractive or repulsive) present in the solution. The measurement is direct with osmotic pressure, whereas with quasi-elastic light scattering, the directly measured coefficient is a combination of thermodynamic and hydrodynamic terms. X-rays, which require more sophisticated equipment such as synchrotron radiation facilities, can provide more detailed information on the interparticle potentials when models are used. At low ionic strength, two potentials were found necessary to account for the temperature and pH phase diagram as a function of protein concentration. The first potential is the van der Waals attractive potential that was previously shown to account for the fluid-fluid phase separation at low temperature. The second potential is an electrostatic coulombic repulsive potential which is a function of the protein charge and thus of the pH. The interaction trail could be followed at protein concentrations as low as 10 mg ml(-1). The results as a whole are expected to be valid for all compact low molecular weight proteins at low ionic strength.

The early stages of the crystallization process of porcine pancreatic alpha-amylase were investigated by quasi-elastic light scattering. It is shown that at 288 and 293 K the diffusion coefficient does not monotonically change with increasing protein concentration but passes through a maximum at 10 mg ml(-1). In supersaturated solutions, prior to nucleation, the protein is strictly monodisperse. Nucleation induces the formation of aggregates and a polydispersity of, for example, 18% for an initial supersaturation C/C(e) = 5.8. Monodispersity is restored after the nuclei have grown and partially consumed the solute. On the other hand, polydispersity increases up to 20% at 298 K if the protein concentration decreases to 3-4 mg ml(-1), values at which the solutions are under-saturated. When the protein concentration exceeds 5-6 mg ml(-1) the protein becomes monodisperse again. These results, confirmed by those of another system we are studying (bovine pancreatic trypsin inhibitor), are at variance with the statements that supersaturation is always at the origin of aggregation and polydispersity, and that in undersaturated solutions the diffusion coefficient should remain constant for obtaining crystals once the solutions are supersaturated.