Usefulness of the dietary phytic acid/ zinc molar ratio as an index of zinc bioavailability to rats and humans

Evidence suggests that rats can tolerate a dietary phytate/Zn molar ratio greater than 15 if the dietary Zn concentration is high. High dietary Ca exacerbates the effect of phytic acid on Zn utilization by rats. In a short term (15 d) balance trial with adult men, we observed slightly greater Zn balance when whole compared to dephytinized wheat bran was consumed (molar ratios 12 and 1.2, respectively). There was, however, greater fecal excretion of Zn during the first 5 d whole bran was consumed. In a second study, Na phytate was the major source of phytic acid and Zn balance was less when the phytate/Zn molar ratio was greater than 16 compared to 4. The difference was not significant, however, and there was evidence of physiological adjustments to maintain homeostasis when the high ratio diet was consumed. Mean Zn intake averaged 17 mg (0.26 mmole) and 11 mg (0.17 mmole) daily for the bran and Na phytate studies, respectively. The level of Zn intake may influence the response of humans to varying phytate/Zn ratios. Comparison of isotope retention studies and the balance data is discussed. Some information on the relationship of dietary Ca to the phytate/Zn effect in human diets is gathered from current literature. The phytate/Zn molar ratio is a useful index of Zn bioavailability.     

Contractility of rat testicular seminiferous tubules in vitro: Prostaglandin F1α and indomethacin

The frequency of spontaneous in vitro contractions of seminiferous tubules of the rat appeared to be increased in a dose-dependent manner by prostaglandin F_1α. PGF_1α treatment increased the tonus of the smooth muscle cells in the wall of the tubules as indicated by a reduction in the diameter of the tubules. When the tubules were rinsed successively with fresh Tyrode's solution, the contractile frequency was diminished. Returning the original bathing medium to the tubules restored their contractile frequency, as did treatment of the rinsed tubules with PGF_1α (10⁻⁷ M). Pre-injecting the rats with indomethacin tended to reduce the contractile frequency of the extirpated tubules. Treating the tubules with a solution of indomethacin for 90 min. in vitro was more effective than pretreatment in vivo in reducing contractile frequency, but a combination of these two procedures produced the greatest inhibition. PGF_1α restored the contractile frequency of the indomethacin-treated tubules. Our results indicate that PGs modulate the in vitro contractility of the tubules.     

The effect of an ilvA Mu phage insertion on ilv gene expression in Escherichia coli K-12

Physiological analysis of an E. coli K-12 strain carrying a Mu phage integrated into the ilvA structural gene shows that there is a polar affect on ilvD gene expression, whereas, the ilvE gene maintains a normal multivalent regulation response. It was also demonstrated that the ilvC and ilvB genes can be derepressed and repressed in response to ilv multivalent control. These experiments demonstrate that the ilvE structural gene can be regulated independently from the ilvA and ilvD structural genes and that the ilvC structural gene does not require the complete ilvA gene product (threonine deaminase) for its induction.     

Preparation of L-tyrosine-ring- 14 C, L-DOPA-ring- 14 C and related metabolites

The reversibility of the tyrosine phenol-lyase reaction has been utilized to develop a simple system in which phenol-¹⁴C is incorporated into l-tyrosine in high yield. By use of mushroom tyrosinase, catechol-¹⁴C can be prepared from phenol-¹⁴C and l-DOPA-¹⁴C from l-tyrosine-¹⁴C. Catechol-¹⁴C can also be incorporated into l-DOPA-¹⁴C by use of tyrosine phenol-lyase, giving the possibility of preparing DOPA with two labeling patterns in the ring when starting with phenol-¹⁴C. Two further tyrosine metabolites, para-coumaric acid and homogentisic acid, have also been enzymatically prepared with ¹⁴C in the ring.     

Sulphate activation in higher plants

Summary ATP-sulphurylase was detected in extracts of roots and leaves of several species of higher plant. The enzyme occurs in the supernatant fraction, has a pH optimum of 8.0 and an absolute requirement for Mg²⁺ ions. Sulphurylase activity is inhibited by selenate and molybdate but not by cysteine, methionine, glutathione, or thiol reagents. The synthesis of sulphurylase by turnip, lettuce, tomato, and Lemna plants grown under aseptic conditions is neither induced by sulphate nor repressed by cystine, and in the latter respect sulphate activation in plants differs from than in micro-organisms. APS-kinase could not be detected in extracts of any tissue although its product was stable under the conditions used. Sulphate reduction in higher plants may thus proceed via adenylysulphate and not via phosphoadenylylsulphate as in many micro-organisms.